huschi.net

S-NAIL(1)                 BSD General Commands Manual                S-NAIL(1)
NAME
     S-nail [v14.9.23] -- send and receive Internet mail
SYNOPSIS
     s-nail [-DdEFinv~#] [-: spec] [-A account] [:-a attachment:]
            [:-b bcc-addr:] [:-C "field: body":] [:-c cc-addr:]
            [-M type | -m file | -q file | -t] [-r from-addr]
            [:-S var[=value]:] [-s subject] [:-T "field: addr":] [:-X cmd:]
            [:-Y cmd:] [-.] :to-addr: [-- :mta-option:]
     s-nail [-DdEeHiNnRv~#] [-: spec] [-A account] [:-C "field: body":]
            [-L spec] [-r from-addr] [:-S var[=value]:] [-u user] [:-X cmd:]
            [:-Y cmd:] [-- :mta-option:]
     s-nail [-DdEeHiNnRv~#] [-: spec] [-A account] [:-C "field: body":] -f
            [-L spec] [-r from-addr] [:-S var[=value]:] [:-X cmd:] [:-Y cmd:]
            [file] [-- :mta-option:]
     s-nail -h | --help
     s-nail -V | --version
DESCRIPTION
           Note: S-nail (S-nail) will see major changes in v15.0 (circa 2022).
           Some backward incompatibilities cannot be avoided.  COMMANDS change
           to  Shell-style  argument  quoting,  and  shell metacharacters will
           become (more) meaningful.  Some commands accept  new  syntax  today
           via  wysh  (Command  modifiers).  Behaviour is flagged [v15-compat]
           and [no v15-compat], setting v15-compat (INTERNAL  VARIABLES)  will
           choose  new behaviour when applicable; giving it a value makes wysh
           an implied default.  [Obsolete] flags what will vanish.
           Warning! v15-compat (with value) will be a default in v14.10.0!
     S-nail provides a simple and friendly environment for sending and receiv-
     ing mail.  It is intended to provide the functionality of the POSIX
     mailx(1) command, but is MIME capable and optionally offers extensions
     for line editing, S/MIME, SMTP and POP3, among others.  S-nail divides
     incoming mail into its constituent messages and allows the user to deal
     with them in any order.  It offers many COMMANDS and INTERNAL VARIABLES
     for manipulating messages and sending mail.  It provides the user simple
     editing capabilities to ease the composition of outgoing messages, and
     increasingly powerful and reliable non-interactive scripting capabili-
     ties.
   Options
     -: spec, --resource-files=..
            Controls loading of (as via source) Resource files: spec is parsed
            case-insensitively, the letter `s' corresponds to the system wide
            s-nail.rc, `u' the user's personal file ~/.mailrc.  The (original)
            system wide resource is also compiled-in, accessible via `x'.  The
            letters `-' and `/' disable usage of resource files.  Order mat-
            ters, default is `su'.  This option overrides -n.
     -A name, --account=..
            Activate user account name after program startup is complete
            (resource files loaded, only -X commands are to be executed), and
            switch to its primary system mailbox (most likely the inbox).  If
            activation fails the program exits if used non-interactively, or
            if any of errexit or posix are set.
     -a file[=input-charset[#output-charset]], --attach=..
            (Send mode) Attach file.  For (Compose mode) opportunities refer
            to ~@ and ~^.  file is subject to tilde expansion (see Filename
            transformations and folder); if it is not accessible but contains
            a `=' character, anything before the last `=' will be used as the
            filename, anything thereafter as a character set specification, as
            shown.
            If only an input character set is specified, the input side is
            fixed, and no character set conversion will be applied; an empty
            or the special string hyphen-minus `-' is taken for ttycharset
            (the default).  If an output character set has also been specified
            the desired conversion is performed immediately, not considering
            file type and content, except for an empty string or hyphen-minus
            `-', which select the default conversion algorithm (see Character
            sets): no immediate conversion is performed, file and its contents
            will be MIME-classified (HTML mail and MIME attachments, The
            mime.types files) first -- only the latter mode is available
            unless features includes `,+iconv,'.
     -B     ([Obsolete]: S-nail will always use line-buffered output, to gain
            line-buffered input even in batch mode enable batch mode via -#.)
     -b addr, --bcc=..
            (Send mode) Send a blind carbon copy to recipient addr.  The
            option may be used multiple times.  Also see the section On
            sending mail, and non-interactive mode.
     -C "field: body", --custom-header=..
            Create a custom header which persists for an entire session.  A
            custom header consists of the field name followed by a colon `:'
            and the field content body, for example `-C "Blah: Neminem laede;
            imo omnes, quantum potes, juva"'.  Standard header field names
            cannot be overwritten by custom headers.  Runtime adjustable cus-
            tom headers are available via the variable customhdr, and in (Com-
            pose mode) ~^, one of the COMMAND ESCAPES, as well as digmsg are
            the most flexible and powerful options to manage message headers.
            This option may be used multiple times.
     -c addr, --cc=..
            (Send mode) Just like -b, except it places the argument in the
            list of carbon copies.
     -D, --disconnected
            [Option] Startup with disconnected set.
     -d, --debug
            Enter a debug-only sandbox mode by setting the internal variable
            debug; the same can be achieved via `-S debug' or `set debug'.
            Also see -v.
     -E, --discard-empty-messages
            (Send mode) set skipemptybody and thus discard messages with an
            empty message part body, successfully.
     -e, --check-and-exit
            Just check if mail is present (in the system inbox or the one
            specified via -f): if yes, return an exit status of zero, a non-
            zero value otherwise.  To restrict the set of mails to consider in
            this evaluation a message specification can be added with the
            option -L.  Quickrun: does not open an interactive session.
     -F     (Send mode) Save the message to send in a file named after the
            local part of the first recipient's address (instead of in
            record).
     -f, --file
            Read in the contents of the user's secondary mailbox MBOX (or the
            specified file) for processing; when S-nail is quit, it writes
            undeleted messages back to this file (but be aware of the hold
            option).  The optional file argument will undergo some special
            Filename transformations (as via folder).  Note that file is not
            an argument to the flag -f, but is instead taken from the command
            line after option processing has been completed.  In order to use
            a file that starts with a hyphen-minus, prefix with a relative
            path, as in `./-hyphenbox.mbox'.
     -H, --header-summary
            Display a summary of headers for the given folder (depending on
            -u, inbox or MAIL, or as specified via -f), then exit.  A config-
            urable summary view is available via the option -L.  This mode
            does not honour showlast.  Quickrun: does not open an interactive
            session.
     -h, --help
            Show a brief usage summary; use --long-help for a list long
            options.
     -i     set ignore to ignore tty interrupt signals.
     -L spec, --search=..
            Display a summary of headers of all messages that match the given
            spec in the folder found by the same algorithm used by -H, then
            exit.  See the section Specifying messages for the format of spec.
            This mode does not honour showlast.
            If the -e option has been given in addition no header summary is
            produced, but S-nail will instead indicate via its exit status
            whether spec matched any messages (`0') or not (`1'); note that
            any verbose output is suppressed in this mode and must instead be
            enabled explicitly (see -v).  Quickrun: does not open an interac-
            tive session.
     -M type
            (Send mode) Will flag standard input with the MIME `Content-Type:'
            set to the given known type (HTML mail and MIME attachments, The
            mime.types files) and use it as the main message body.  [v15 be-
            haviour may differ] Using this option will bypass processing of
            message-inject-head and message-inject-tail.  Also see -q, -m, -t.
     -m file
            (Send mode) MIME classify the specified file and use it as the
            main message body.  [v15 behaviour may differ] Using this option
            will bypass processing of message-inject-head and
            message-inject-tail.  Also see -q, -M, -t.
     -N, --no-header-summary
            inhibit the initial display of message headers when reading mail
            or editing a mailbox folder by calling unset for the internal
            variable header.
     -n     Standard flag that inhibits reading the system wide s-nail.rc upon
            startup.  The option -: allows more control over the startup
            sequence; also see Resource files.
     -q file, --quote-file=..
            (Send mode) Initialize the message body with the contents of file,
            which may be standard input `-' only in non-interactive context.
            Also see -M, -m, -t.
     -R, --read-only
            Any mailbox folder aka folder opened will be in read-only mode.
     -r from-addr, --from-address=..
            The RFC 5321 reverse-path used for relaying and delegating mes-
            sages to its destination(s), for example to report delivery
            errors, is normally derived from the address which appears in the
            from header (or, if that contains multiple addresses, in sender).
            A file-based aka local executable mta (Mail-Transfer-Agent), how-
            ever, instead uses the local identity of the initiating user.
            When this command line option is used the given single addressee
            from-addr will be assigned to the internal variable from, but in
            addition the command line option -f from-addr will be passed to a
            file-based mta whenever a message is sent.  Shall from-addr
            include a user name the address components will be separated and
            the name part will be passed to a file-based mta individually via
            -F name.  Even though not a recipient the `shquote' expandaddr
            flag is supported.
            If an empty string is passed as from-addr then the content of the
            variable from (or, if that contains multiple addresses, sender)
            will be evaluated and used for this purpose whenever the file-
            based mta is contacted.  By default, without -r that is, neither
            -f nor -F command line options are used when contacting a file-
            based MTA, unless this automatic deduction is enforced by setting
            the internal variable r-option-implicit.
            Remarks: many default installations and sites disallow overriding
            the local user identity like this unless either the MTA has been
            configured accordingly or the user is member of a group with spe-
            cial privileges.  Passing an invalid address will cause an error.
     -S var[=value], --set=..
            set (or, with a prefix string `no', as documented in INTERNAL
            VARIABLES, unset) variable and optionally assign value, if sup-
            ported; [v15 behaviour may differ] the entire expression is evalu-
            ated as if specified within dollar-single-quotes (see Shell-style
            argument quoting) if the internal variable v15-compat is set.  If
            the operation fails the program will exit if any of errexit or
            posix are set.  Settings established via -S cannot be changed from
            within Resource files or an account switch initiated by -A.  They
            will become mutable again before commands registered via -X are
            executed.
     -s subject, --subject=..
            (Send mode) Specify the subject of the message to be sent.  New-
            line (NL) and carriage-return (CR) bytes are invalid and will be
            normalized to space (SP) characters.
     -T "field: addr", --target=..
            (Send mode) Add addr to the list of receivers targeted by field,
            for now supported are only `bcc', `cc', `fcc', and `to'.  Field
            and body (address) are separated by a colon `:' and optionally
            blank (space, tabulator) characters.  The `shquote' expandaddr
            flag is supported.  addr is parsed like a message header address
            line, as if it would be part of a template message fed in via -t,
            and the same modifier suffix is supported.  This option may be
            used multiple times.
     -t, --template
            (Send mode) The text message given (on standard input) is expected
            to contain, separated from the message body by an empty line, one
            or multiple plain text message headers.  [v15 behaviour may dif-
            fer] Readily prepared MIME mail messages cannot be passed.  Head-
            ers can span multiple consecutive lines if follow lines start with
            any amount of whitespace.  A line starting with the number sign
            `#' in the first column is ignored.  Message recipients can be
            given via the message headers `To:', `Cc:', `Bcc:' (the `?single'
            modifier enforces treatment as a single addressee, for example
            `To?single: exa, <m@ple>') or `Fcc:', they will be added to any
            recipients specified on the command line, and are likewise subject
            to expandaddr validity checks.  If a message subject is specified
            via `Subject:' then it will be used in favour of one given on the
            command line.
            More optional headers are `Reply-To:' (possibly overriding
            reply-to), `Sender:' (sender), `From:' (from and / or option -r).
            `Message-ID:', `In-Reply-To:', `References:' and
            `Mail-Followup-To:', by default created automatically dependent on
            message context, will be used if specified (a special address mas-
            sage will however still occur for the latter).  Any other custom
            header field (also see -C, customhdr and ~^) is passed through
            entirely unchanged, and in conjunction with the options -~ or -#
            it is possible to embed COMMAND ESCAPES.  Also see -M, -m, -q.
     -u user, --inbox-of=..
            Initially read the primary system mailbox of user, appropriate
            privileges presumed; effectively identical to `-f %user'.
     -V, --version
            Show S-nails version and exit.  The command version will also show
            the list of features: `$ s-nail -:/ -Xversion -Xx'.
     -v, --verbose
            sets the internal variable verbose to enable logging of informa-
            tional context messages.  (Increases level of verbosity when used
            multiple times.)  Also see -d.
     -X cmd, --startup-cmd=..
            Add the given (or multiple for a multiline argument) cmd to a list
            of commands to be executed before normal operation starts.  The
            commands will be evaluated as a unit, just as via source.  Corre-
            lates with -# and errexit.
     -Y cmd, --cmd=..
            Add the given (or multiple for a multiline argument) cmd to a list
            of commands to be executed after normal operation has started.
            The commands will be evaluated successively in the given order,
            and as if given on the program's standard input -- before interac-
            tive prompting begins in interactive mode, after standard input
            has been consumed otherwise.
     -~, --enable-cmd-escapes
            Enable COMMAND ESCAPES in (Compose mode) even in non-interactive
            use cases.  This can for example be used to automatically format
            the composed message text before sending the message:
                  $ ( echo 'line    one. Word.     Word2.';\
                      echo '~| /usr/bin/fmt -tuw66' ) |\
                    LC_ALL=C s-nail -d~:/ -Sttycharset=utf-8 bob AT exam.ple
     -#, --batch-mode
            Enables batch mode: standard input is made line buffered, the com-
            plete set of (interactive) commands is available, processing of
            COMMAND ESCAPES is enabled in Compose mode, and diverse INTERNAL
            VARIABLES are adjusted for batch necessities, exactly as if done
            via -S: emptystart, noerrexit, noheader, noposix, quiet, sendwait,
            typescript-mode as well as MAIL, MBOX and inbox (the latter three
            to /dev/null).  Also, the values of COLUMNS and LINES are looked
            up, and acted upon.  The following prepares an email message in a
            batched dry run:
                  $ for name in bob alice AT exam.ple lisa AT exam.ple; do
                      printf 'mail %s\n~s ubject\nText\n~.\n' "${name}"
                    done |
                    LC_ALL=C s-nail -#:x -Smta=test \
                      -X'alias bob bob AT exam.ple'
     -., --end-options
            This flag forces termination of option processing in order to pre-
            vent ``option injection'' (attacks).  It also forcefully puts
            S-nail into send mode, see On sending mail, and non-interactive
            mode.
     If the setting of expandargv allows their recognition all mta-option
     arguments given at the end of the command line after a `--' separator
     will be passed through to a file-based mta (Mail-Transfer-Agent) and per-
     sist for the entire session.  expandargv constraints do not apply to the
     content of mta-arguments.  Command line receiver address handling sup-
     ports the `shquote' constraint of expandaddr, for more please see On
     sending mail, and non-interactive mode.
           $ s-nail -#:/ -X 'addrcodec enc Hey, ho <silver@go>' -Xx
   A starter
     S-nail is a direct descendant of BSD Mail, itself a successor to the
     Research UNIX mail which ``was there from the start'' according to
     HISTORY.  It thus represents the user side of the UNIX mail system,
     whereas the system side (Mail-Transfer-Agent, MTA) was traditionally
     taken by sendmail(8) (and most MTAs provide a binary of this name for
     compatibility reasons).  If the [Option]al SMTP mta is included in the
     features of S-nail then the system side is not a mandatory precondition
     for mail delivery.
     S-nail strives for compliance with the POSIX mailx(1) standard, but
     posix, one of the INTERNAL VARIABLES, or its ENVIRONMENTal equivalent
     POSIXLY_CORRECT, needs to be set to adjust behaviour to be almost on par.
     Almost, because there is one important difference: POSIX Shell-style
     argument quoting is ([v15 behaviour may differ] increasingly) used
     instead of the Old-style argument quoting that the standard documents,
     which is believed to be a feature.  The builtin as well as the (default)
     global s-nail.rc Resource files already bend the standard imposed set-
     tings a bit.
     For example, hold and keepsave are set in order to suppress the automatic
     moving of messages to the secondary mailbox MBOX that would otherwise
     occur (see Message states), and keep to not remove empty system MBOX
     mailbox files (or all empty such files in posix mode) to avoid mangling
     of file permissions when files eventually get recreated.
     To enter interactive mode even if the initial mailbox is empty emptystart
     is set, editheaders to allow editing of headers as well as fullnames to
     not strip down addresses in Compose mode, and quote to include the mes-
     sage that is being responded to when replying, which is indented by an
     indentprefix that also deviates from standard imposed settings.
     mime-counter-evidence is fully enabled, too.  It sets followup-to-honour
     and reply-to-honour to comply with reply address desires.
     Credentials and other settings are easily addressable by grouping them
     via account.  The file mode creation mask can be managed with umask.
     Files and shell pipe output can be sourced for evaluation, also during
     startup from within the Resource files.  Informational context can be
     available by setting verbose or debug (as via -v, -d).
   On sending mail, and non-interactive mode
     To send a message to one or more people, using a local or built-in mta
     (Mail-Transfer-Agent) transport to actually deliver the generated mail
     message, S-nail can be invoked with arguments which are the names of peo-
     ple to whom the mail will be sent, and the command line options -b and -c
     can be used to add (blind) carbon copy receivers:
           # Via test MTA
           $ echo Hello, world | s-nail -:/ -Smta=test -s test $LOGNAME
           # Via sendmail(1) MTA
           $ </dev/null s-nail -:x -s test $LOGNAME
           # Debug dry-run mode:
           $ </dev/null LC_ALL=C s-nail -d -:/ \
              -Sttycharset=utf8 -Sfullnames \
              -b bcc AT exam.ple -c cc AT exam.ple -. \
              '(Lovely) Bob <bob AT exam.ple>' eric AT exam.ple
           # With SMTP (no real sending due to -d debug dry-run)
           $ LC_ALL=C s-nail -d -:/ -Sv15-compat -Sttycharset=utf8 \
               -S mta=smtps://mylogin AT exam.ple:465 -Ssmtp-auth=none \
               -S from=scriptreply AT exam.ple \
               -a /etc/mail.rc --end-options \
               eric AT exam.ple < /tmp/letter.txt
     Email addresses and plain user names are subject to alternates filtering,
     names only are first expanded through alias and mta-aliases.  An address
     in angle brackets consisting only of a valid local user `<name>' will be
     converted to a fully qualified address if either hostname is not set, or
     set to a non-empty value; if set to the empty value the conversion is
     left up to the mta.  By setting expandaddr fine-grained control of recip-
     ient address types other than user names and network addresses is possi-
     ble.  Recipients are classified as follows: any name that starts with a
     vertical bar `|' character specifies a command pipe - the command string
     following the `|' is executed and the message is sent to its standard
     input; likewise, any name that consists only of hyphen-minus `-' or
     starts with the character solidus `/' or the character sequence dot
     solidus `./' is treated as a file, regardless of the remaining content.
     Any other name which contains a commercial at `@' character is a network
     address; Any other name which starts with a plus sign `+' character is a
     mailbox name; Any other name which contains a solidus `/' character but
     no exclamation mark `!' or percent sign `%' character before is also a
     mailbox name; What remains is treated as a network address.  This classi-
     fication can be avoided by using a `Fcc:' header, see Compose mode.
           $ echo bla | s-nail -Sexpandaddr -s test ./mbox.mbox
           $ echo bla | s-nail -Sexpandaddr -s test '|cat >> ./mbox.mbox'
           $ echo safe | LC_ALL=C \
               s-nail -:/ -Smta=test -Sv15-compat -Sttycharset=utf8 \
                 --set mime-force-sendout --set fullnames \
                 -S expandaddr=fail,-all,+addr,failinvaddr -s test \
                 --end-options 'Imagine John <cold AT turk.ey>'
     Before messages are sent they undergo editing in Compose mode.  But many
     settings are static and can be set more generally.  The envelope sender
     address for example is defined by from, explicitly defining an originat-
     ing hostname may be desirable, especially with the built-in SMTP Mail-
     Transfer-Agent mta.  Character sets for outgoing message and MIME part
     content are configurable via sendcharsets, whereas input data is assumed
     to be in ttycharset.  Message data will be passed over the wire in a
     mime-encoding, and MIME parts aka attachments need a mimetype, usually
     taken out of The mime.types files.  Saving copies of sent messages in a
     record mailbox may be desirable - as for most mailbox folder targets
     Filename transformations will be performed.
     For the purpose of arranging a complete environment of settings that can
     be switched to with a single command or command line option there are
     accounts.  Alternatively a flat configuration could be possible, making
     use of so-called variable chains which automatically pick `USER@HOST' or
     `HOST' context-dependent variants some variables support: for example
     addressing `Folder pop3://yaa AT exam.ple' would find
     pop3-no-apop-yaa AT exam.ple, pop3-no-apop-exam.ple and pop3-no-apop in
     order.  For more please see On URL syntax and credential lookup and
     INTERNAL VARIABLES.
     To avoid environmental noise scripts should create a script-local envi-
     ronment, ideally with the command line options -: to disable configura-
     tion files in conjunction with repetitions of -S to specify variables:
           $ env LC_ALL=C s-nail -:/ \
               -Sv15-compat \
               -Sttycharset=utf-8 -Smime-force-sendout \
               -Sexpandaddr=fail,-all,failinvaddr \
               -S mta=smtps://mylogin AT exam.ple:465 -Ssmtp-auth=login \
               -S from=scriptreply AT exam.ple \
               -s 'Subject to go' -a attachment_file \
               -Sfullnames -. \
               'Recipient 1 <rec1 AT exam.ple>' rec2 AT exam.ple \
               < content_file
     As shown, scripts producing messages can ``fake'' a locale environment,
     the above specifies the all-compatible 7-bit clean LC_ALL ``C'', but will
     nonetheless take and send UTF-8 in the message text by using ttycharset.
     If character set conversion is compiled in (features includes the term
     `,+iconv,') invalid (according to ttycharset) character input data would
     normally cause errors; setting mime-force-sendout will instead, as a last
     resort, classify the input as binary data, and therefore allow message
     creation to be successful.  (Such content can then be inspected either by
     installing a pipe-TYPE/SUBTYPE handler for `application/octet-stream', or
     possibly automatically through mime-counter-evidence).
     In interactive mode, introduced soon, messages can be sent by calling the
     mail command with a list of recipient addresses:
           $ s-nail -:/ -Squiet -Semptystart -Sfullnames -Smta=test
           "/var/spool/mail/user": 0 messages
           ? mail "Recipient 1 <rec1 AT exam.ple>", rec2 AT exam.ple
           ...
           ? # Will do the right thing (tm)
           ? m rec1 AT exam.ple rec2 AT exam.ple
   Compose mode
     If standard input is a terminal rather than the message to be sent, the
     user is expected to type in the message contents.  In compose mode lines
     beginning with the character `~' (in fact the value of escape) are spe-
     cial - these are so-called COMMAND ESCAPES which can be used to read in
     files, process shell commands, add and edit attachments and more.  For
     example ~v or ~e will start the VISUAL text EDITOR, respectively, to
     revise the message in its current state, ~h allows editing of the most
     important message headers, with the potent ~^ custom headers can be cre-
     ated, for example (more specifically than with -C and customhdr).
     [Option]ally ~? gives an overview of most other available command
     escapes.
     To create file-carbon-copies the special recipient header `Fcc:' may be
     used as often as desired, for example via ~^.  Its entire value (or body
     in standard terms) is interpreted as a folder target, after having been
     subject to Filename transformations: this is the only way to create a
     file-carbon-copy without introducing an ambiguity regarding the interpre-
     tation of the address, file names with leading vertical bars or commer-
     cial ats can be used.  Like all other recipients `Fcc:' is subject to the
     checks of expandaddr.  Any local file and pipe command addressee honours
     the setting of mbox-fcc-and-pcc.
     Once finished with editing the command escape ~. (see there) will call
     hooks, insert automatic injections and receivers, leave compose mode and
     send the message once it is completed.  Aborting letter composition is
     possible with either of ~x or ~q, the latter of which will save the mes-
     sage in the file denoted by DEAD unless nosave is set.  And unless
     ignoreeof is set the effect of ~. can also be achieved by typing end-of-
     transmission (EOT) via `control-D' (`^D') at the beginning of an empty
     line, and ~q is always reachable by typing end-of-text (ETX) twice via
     `control-C' (`^C').
     The compose mode hooks on-compose-enter, on-compose-splice,
     on-compose-leave and on-compose-cleanup may be set to defined macros and
     provide reliable and increasingly powerful mechanisms to perform auto-
     mated message adjustments dependent on message context, for example addi-
     tion of message signatures (message-inject-head, message-inject-tail) or
     creation of additional receiver lists (also by setting autocc, autobcc).
     To achieve that the command digmsg may be used in order to query and
     adjust status of message(s).  The splice hook can also make use of
     COMMAND ESCAPES.  ([v15 behaviour may differ] The compose mode hooks work
     for forward, mail, reply and variants; resend and Resend only provide the
     hooks on-resend-enter and on-resend-cleanup, which are pretty restricted
     due to the nature of the operation.)
   On reading mail, and more on interactive mode
     When invoked without addressees S-nail enters interactive mode in which
     mails may be read.  When used like that the user's system inbox (for more
     on mailbox types please see the command folder) is read in and a one line
     header of each message therein is displayed if the variable header is
     set.  The visual style of this summary of headers can be adjusted through
     the variable headline and the possible sorting criterion via autosort.
     Scrolling through screenfuls of headers can be performed with the command
     z.  If the initially opened mailbox is empty S-nail will instead exit
     immediately (after displaying a message) unless the variable emptystart
     is set.
     At the prompt the command list will give a listing of all available com-
     mands and help will [Option]ally give a summary of some common ones.  If
     the [Option]al documentation strings are available (see features) one can
     type `help X' (or `?X') and see the actual expansion of `X' and what its
     purpose is, i.e., commands can be abbreviated (note that POSIX defines
     some abbreviations, so that the alphabetical order of commands does not
     necessarily relate to the abbreviations; it is however possible to define
     overwrites with commandalias).  These commands can also produce a more
     verbose output.
     Messages are given numbers (starting at 1) which uniquely identify mes-
     sages; the current message - the ``dot'' - will either be the first new
     message, or the first unread message, or the first message of the mail-
     box; the internal variable showlast will instead cause usage of the last
     message for this purpose.  The command headers will display a screenful
     of header summaries containing the ``dot'', whereas from will display
     only the summaries of the given messages, defaulting to the ``dot''.
     Message content can be displayed with the command type (`t', alias
     print).  Here the variable crt controls whether and when S-nail will use
     the configured PAGER for display instead of directly writing to the user
     terminal screen, the sole difference to the command more, which will
     always use the PAGER.  The command top will instead only show the first
     toplines of a message (maybe even compressed if topsqueeze is set).  Mes-
     sage display experience may improve by setting and adjusting
     mime-counter-evidence, and also see HTML mail and MIME attachments.
     By default the current message (``dot'') is displayed, but like with many
     other commands it is possible to give a fancy message specification (see
     Specifying messages), for example `t:u' will display all unread messages,
     `t.' will display the ``dot'', `t 1 5' will type the messages 1 and 5, `t
     1-5' will type the messages 1 through 5, and `t-' and `t+' will display
     the previous and the next message, respectively.  The command search (a
     more substantial alias for from) will display a header summary of the
     given message specification list instead of their content; the following
     will search for subjects:
           ? from '@Some subject to search for'
     In the default setup all header fields of a message will be typed, but
     fields can be white- or blacklisted for a variety of applications by
     using the command headerpick, e.g., to restrict their display to a very
     restricted set for type: `headerpick type retain from to cc subject'.  In
     order to display all header fields of a message regardless of currently
     active ignore or retain lists, use the commands Type and Top; Show will
     show the raw message content.  Note that historically the global
     s-nail.rc not only adjusts the list of displayed headers, but also sets
     crt.  ([v15 behaviour may differ] A yet somewhat restricted) Reliable
     scriptable message inspection is available via digmsg.
     Dependent upon the configuration a line editor (see the section On
     terminal control and line editor) aims at making the user experience with
     the many COMMANDS a bit nicer.  When reading the system inbox, or when -f
     (or folder) specified a mailbox explicitly prefixed with the special `%:'
     modifier (to propagate it to a primary system mailbox), then messages
     which have been read (see Message states) will be automatically moved to
     a secondary mailbox, the user's MBOX file, when the mailbox is left,
     either by changing the active mailbox or by quitting S-nail - this auto-
     matic moving from a system- or primary- to the secondary mailbox is not
     performed when the variable hold is set.  Messages can also be explicitly
     moved to other mailboxes, whereas copy keeps the original message.  write
     can be used to write out data content of specific parts of messages.
     After examining a message the user can reply `r' to the sender and all
     recipients (which will also be placed in `To:' unless recipients-in-cc is
     set), or Reply `R' exclusively to the sender(s).  To comply with with the
     receivers desired reply address the quadoptions followup-to-honour and
     reply-to-honour should usually be set.  The commands Lreply and Lfollowup
     know how to apply a special addressee massage, see Mailing lists.  Depen-
     dent on the presence and value of quote the message being replied to will
     be included in a quoted form.  forwarding a message will allow editing
     the new message: the original message will be contained in the message
     body, adjusted according to headerpick.  It is possible to resend or
     Resend messages: the former will add a series of `Resent-' headers,
     whereas the latter will not; different to newly created messages editing
     is not possible and no copy will be saved even with record unless the
     additional variable record-resent is set.  When sending, replying or for-
     warding messages comments and full names will be stripped from recipient
     addresses unless the internal variable fullnames is set.
     Of course messages can be delete `d', and they can spring into existence
     again via undelete, or when the S-nail session is ended via the exit or
     xit commands to perform a quick program termation.  To end a mail pro-
     cessing session regularly and perform a full program exit one may issue
     the command quit.  It will, among others, move read messages to the
     secondary mailbox MBOX as necessary, discard deleted messages in the cur-
     rent mailbox, and update the [Option]al (see features) line editor
     history-file.  By the way, whenever the main event loop is about to look
     out for the next input line it will trigger the hook on-main-loop-tick.
   HTML mail and MIME attachments
     HTML-only messages become more and more common, and many messages come
     bundled with a bouquet of MIME (Multipurpose Internet Mail Extensions)
     parts and attachments.  To get a notion of MIME types there is a built-in
     default set, onto which the content of The mime.types files will be added
     (as configured and allowed by mimetypes-load-control).  Types can also
     become registered and listed with the command mimetype.  To improve
     interaction with the faulty MIME part declarations of real life
     mime-counter-evidence will allow verification of the given assertion, and
     the possible provision of an alternative, better MIME type.  Note plain
     text parts will always be preferred in `multipart/alternative' MIME mes-
     sages unless mime-alternative-favour-rich is set.
     Whereas a simple HTML-to-text filter for displaying HTML messages is
     [Option]ally supported (indicated by `,+filter-html-tagsoup,' in
     features), MIME types other than plain text cannot be handled directly.
     To deal with specific non-text MIME types or file extensions programs
     need to be registered which either prepare (re-)integrable plain text
     versions of their input (a mode which is called copiousoutput), or dis-
     play the content externally, for example in a graphical window: the lat-
     ter type is only considered by and for the command mimeview.
     To install a handler program for a MIME type an according
     pipe-TYPE/SUBTYPE variable needs to be set; to define a handler for a
     file extension pipe-EXTENSION can be used - these handlers take prece-
     dence.  [Option]ally mail user agent configuration is supported (see The
     Mailcap files), and will be queried for display or quote handlers after
     the former ones.  Type-markers registered via mimetype are the last pos-
     sible source for information how to handle a MIME type.
     For example, to display HTML messages integrated via the text browsers
     lynx(1) or elinks(1), register a MathML MIME type and enable its plain
     text display, and to open PDF attachments in an external PDF viewer,
     asynchronously and with some other magic attached:
           ? if "$features" !% ,+filter-html-tagsoup,
           ?   #set pipe-text/html='?* elinks -force-html -dump 1'
           ?   set pipe-text/html='?* lynx -stdin -dump -force_html'
           ?   # Display HTML as plain text instead
           ?   #set pipe-text/html=?t
           ? endif
           ? mimetype ?t application/mathml+xml mathml
           ? wysh set pipe-application/pdf='?&=? \
               trap "rm -f \"${MAILX_FILENAME_TEMPORARY}\"" EXIT;\
               trap "trap \"\" INT QUIT TERM; exit 1" INT QUIT TERM;\
               mupdf "${MAILX_FILENAME_TEMPORARY}"'
           ? define showhtml {
           ?   \localopts yes
           ?   \set mime-alternative-favour-rich pipe-text/html=?h?
           ?   \type "$@"
           ? }
           ? \commandalias html \\call showhtml
   Mailing lists
     Known or subscribed-to mailing lists may be flagged in the summary of
     headers (headline format character `%L'), and will gain special treatment
     when sending mails: the variable followup-to-honour will ensure that a
     `Mail-Followup-To:' header is honoured when a message is being replied to
     (reply, followup, Lreply, Lfollowup), and followup-to controls creation
     of this header when creating mails, if the necessary user setup (from,
     sender); is available; then, it may also be created automatically, for
     example when list-replying via Lreply or Lfollowup, when followup or
     reply is used and the messages `Mail-Followup-To:' is honoured etc.
     The commands mlist and mlsubscribe manage S-nails notion of which
     addresses are mailing lists.  With the [Option]al regular expression sup-
     port any address which contains any of the magic regular expression char-
     acters (`^[*+?|$'; see re_format(7) or regex(7), dependent on the host
     system) will be compiled and used as one, possibly matching many
     addresses.  It is not possible to escape the ``magic'': in order to match
     special characters as-is, bracket expressions must be used, for example
     `search @subject@'[[]open bracket''.
           ? set followup-to followup-to-honour=ask-yes \
               reply-to-honour=ask-yes
           ? mlist a1 AT b1.c1 a2 AT b2.c2 '.*@lists\.c3$'
           ? mlsubscribe a4 AT b4.c4 exact AT lists.c3
     Known and subscribed lists differ in that for the latter the users
     address is not part of a generated `Mail-Followup-To:'.  There are excep-
     tions, for example if multiple lists are addressed and not all have the
     subscription attribute.  When replying to a message its list address
     (`List-Post:' header) is automatically and temporarily treated like a
     known mlist; dependent on the variable reply-to-honour an existing
     `Reply-To:' is used instead (if it is a single address on the same domain
     as `List-Post:') in order to accept a list administrator's wish that is
     supposed to have been manifested like that.
     For convenience and compatibility with mail programs that do not honour
     the non-standard M-F-T, an automatic user entry in the carbon-copy `Cc:'
     address list of generated message can be created by setting
     followup-to-add-cc.  This entry will be added whenever the user will be
     placed in the `Mail-Followup-To:' list, and is not a regular addressee
     already.  reply-to-swap-in tries to deal with the address rewriting that
     many mailing-lists nowadays perform to work around DKIM / DMARC etc.
     standard imposed problems.
   Signed and encrypted messages with S/MIME
     [Option] S/MIME provides two central mechanisms: message signing and mes-
     sage encryption.  A signed message contains some data in addition to the
     regular text.  The data can be used to verify that the message has been
     sent using a valid certificate, that the sender address matches that in
     the certificate, and that the message text has not been altered.  Signing
     a message does not change its regular text; it can be read regardless of
     whether the recipients software is able to handle S/MIME.  It is thus
     usually possible to sign all outgoing messages if so desired.
     Encryption, in contrast, makes the message text invisible for all people
     except those who have access to the secret decryption key.  To encrypt a
     message, the specific recipients public encryption key must be known.  It
     is therefore not possible to send encrypted mail to people unless their
     key has been retrieved from either previous communication or public key
     directories.  Because signing is performed with private keys, and encryp-
     tion with public keys, messages should always be signed before being
     encrypted.
     A central concept to S/MIME is that of the certification authority (CA).
     A CA is a trusted institution that issues certificates.  For each of
     these certificates it can be verified that it really originates from the
     CA, provided that the CA's own certificate is previously known.  A set of
     CA certificates is usually delivered and installed together with the
     cryptographical library that is used on the local system.  Therefore rea-
     sonable security for S/MIME on the Internet is provided if the source
     that provides that library installation is trusted.  It is also possible
     to use a specific pool of trusted certificates.  If this is desired,
     smime-ca-no-defaults should be set to avoid using the default certificate
     pool, and smime-ca-file and/or smime-ca-dir should be pointed to a
     trusted pool of certificates.  A certificate cannot be more secure than
     the method its CA certificate has been retrieved with.
     This trusted pool of certificates is used by the command verify to ensure
     that the given S/MIME messages can be trusted.  If so, verified sender
     certificates that were embedded in signed messages can be saved locally
     with the command certsave, and used by S-nail to encrypt further communi-
     cation with these senders:
           ? certsave FILENAME
           ? set smime-encrypt-USER@HOST=FILENAME \
               smime-cipher-USER@HOST=AES256
     To sign outgoing messages, in order to allow receivers to verify the ori-
     gin of these messages, a personal S/MIME certificate is required.  S-nail
     supports password-protected personal certificates (and keys), see
     smime-sign-cert.  The section On URL syntax and credential lookup gives
     an overview of the possible sources of user credentials, and S/MIME step
     by step shows examplarily how a private S/MIME certificate can be
     obtained.  In general, if such a private key plus certificate ``pair'' is
     available, all that needs to be done is to set some variables:
           ? set smime-sign-cert=ME AT exam.paired \
               smime-sign-digest=SHA512 \
               smime-sign from=myname AT my.host
     Variables of interest for S/MIME in general are smime-ca-dir,
     smime-ca-file, smime-ca-flags, smime-ca-no-defaults, smime-crl-dir,
     smime-crl-file.  For S/MIME signing of interest are smime-sign,
     smime-sign-cert, smime-sign-include-certs and smime-sign-digest.  Addi-
     tional variables of interest for S/MIME en- and decryption: smime-cipher
     and smime-encrypt-USER@HOST.  Variables of secondary interest may be
     content-description-smime-message and
     content-description-smime-signature.  S/MIME is available if `,+smime,'
     is included in features.
     [v15 behaviour may differ] Note that neither S/MIME signing nor encryp-
     tion applies to message subjects or other header fields yet.  Thus they
     may not contain sensitive information for encrypted messages, and cannot
     be trusted even if the message content has been verified.  When sending
     signed messages, it is recommended to repeat any important header infor-
     mation in the message text.
   On URL syntax and credential lookup
     For accessing protocol-specific resources Uniform Resource Locators (URL,
     RFC 3986) have become omnipresent.  Here they are expected in a
     ``normalized'' variant, not used in data exchange, but only meant as a
     compact, easy-to-use way of defining and representing information in a
     well-known notation; as such they do not conform to any real standard.
     Optional parts are placed in brackets `[]', optional either because there
     also exist other ways to define the information, or because the part is
     protocol specific.  `/path' for example is used by the [Option]al Maildir
     folder type and the IMAP protocol, but not by POP3.  If `USER' and
     `PASSWORD' are included in an URL server specification, URL percent
     encoded (RFC 3986) forms are needed, generable with urlcodec.
           PROTOCOL://[USER[:PASSWORD]@]server[:port][/path]
     Often INTERNAL VARIABLES exist in multiple versions, called ``variable
     chains'' in this document: the plain `variable' as well as
     `variable-HOST' and `variable-USER@HOST'.  If a port was specified `HOST'
     really means `server:port', not `server'.  And this `USER' is never in
     URL percent encoded form.  For example, whether the hypothetical `smtp://
     wings%3Aof AT a.dove' including user and password was used, or whether it
     was `smtp://a.dove' and it came from a different source, to lookup the
     chain tls-config-pairs first `tls-config-pairs-wings:of AT a.dove' is looked
     up, then `tls-config-pairs-a.dove', before finally looking up the plain
     variable.
     The logic to collect (an accounts) credential information is as follows:
     o   A user is always required.  If no `USER' has been given in the URL
         the variables user-HOST and user are looked up.  Afterwards, when
         enforced by the [Option]al variables netrc-lookup-HOST or
         netrc-lookup, The .netrc file of the user will be searched for a
         `HOST' specific entry which provides a `login' name: only unambiguous
         entries are used (one possible matching entry for `HOST').
         If there is still no `USER' then the verified LOGNAME, known to be a
         valid user on the current host, is used.
     o   Authentication: unless otherwise noted the chain
         PROTOCOL-auth-USER@HOST, PROTOCOL-auth-HOST, PROTOCOL-auth is
         checked, falling back to a protocol-specific default as necessary.
     o   If no `PASSWORD' has been given in the URL, then if the `USER' has
         been found through the [Option]al netrc-lookup, that may have also
         provided the password.  Otherwise the chain password-USER@HOST,
         password-HOST, password is looked up.
         Thereafter the (now complete) [Option]al chain
         netrc-lookup-USER@HOST, netrc-lookup-HOST, netrc-lookup is checked,
         if set the netrc cache is searched for a password only (multiple user
         accounts for a single machine may exist as well as a fallback entry
         without user but with a password).
         If at that point there is still no password available, but the (pro-
         tocols') chosen authentication type requires a password, then in
         interactive mode the user will be prompted on the terminal.
     Note: S/MIME verification works relative to the values found in the
     `From:' (or `Sender:') header field(s), which means the values of
     smime-sign, smime-sign-cert, smime-sign-include-certs and
     smime-sign-digest will not be looked up using the `USER' and `HOST'
     chains from above, but instead use the corresponding values from the mes-
     sage that is being worked on.  If no address matches we assume and use
     the setting of from.  In unusual cases multiple and different `USER' and
     `HOST' combinations may therefore be involved - on the other hand those
     unusual cases become possible.  The usual case is as short as:
           set mta=smtp://USER:PASS@HOST smtp-use-starttls \
               smime-sign smime-sign-cert=+smime.pair \
               from=myname AT my.host
     The section EXAMPLES contains complete example configurations.
   Encrypted network communication
     [Option] SSL (Secure Sockets Layer) aka its successor TLS (Transport
     Layer Security) are protocols which aid in securing communication by pro-
     viding a safely initiated and encrypted network connection.  A central
     concept of TLS are certificates: as part of each network connection setup
     a (set of) certificates will be exchanged through which the identity of
     the network peer can be cryptographically verified; if possible the
     TLS/SNI (ServerNameIndication) extension will be enabled to allow servers
     fine-grained control over the certificates being used.  A locally
     installed pool of trusted certificates will then be inspected, and veri-
     fication will succeed if it contains a(n in)direct signer of the pre-
     sented certificate(s).
     The local pool of trusted so-called CA (Certification Authority) certifi-
     cates is usually delivered with and used along the TLS library.  A custom
     pool of trusted certificates can be selected by pointing tls-ca-file
     and/or (with special preparation) tls-ca-dir to the desired location;
     setting tls-ca-no-defaults in addition will avoid additional inspection
     of the default pool.  A certificate cannot be more secure than the method
     its CA certificate has been retrieved with.  For inspection or other pur-
     poses, the certificate of a server (as seen when connecting to it) can be
     fetched with the command tls (port can usually be the protocol name, too,
     and tls-verify is taken into account here):
           $ s-nail -vX 'tls certchain SERVER-URL[:PORT]; x'
     A local pool of CA certificates is not strictly necessary, however,
     server certificates can also be verified via their fingerprint.  For this
     a message digest will be calculated and compared against the variable
     chain tls-fingerprint, and verification will succeed if the fingerprint
     matches.  The message digest (algorithm) can be configured via the vari-
     able chain tls-fingerprint-digest; tls can again be used:
           $ s-nail -X 'wysh set verbose; tls fingerprint SERVER-URL[:PORT]; x'
     It depends on the used protocol whether encrypted communication is possi-
     ble, and which configuration steps have to be taken to enable it.  Some
     protocols, like POP3S, are implicitly encrypted, others, like POP3, can
     upgrade a plain text connection if so requested.  For example, to use the
     `STLS' that POP3 offers (a member of) the variable (chain)
     pop3-use-starttls needs to be set, with convenience via shortcut:
           shortcut encpop1 pop3s://pop1.exam.ple
           shortcut encpop2 pop3://pop2.exam.ple
           set pop3-use-starttls-pop2.exam.ple
           set mta=smtps://smtp.exam.ple:465
           set mta=smtp://smtp.exam.ple smtp-use-starttls
     Normally that is all there is to do, given that TLS libraries try to pro-
     vide safe defaults, plenty of knobs however exist to adjust settings.
     For example certificate verification settings can be fine-tuned via
     tls-ca-flags, and the TLS configuration basics are accessible via
     tls-config-pairs, for example to control protocol versions or cipher
     lists.  In the past hints on how to restrict the set of protocols to
     highly secure ones were indicated, but as of the time of this writing the
     list of protocols or ciphers may need to become relaxed in order to be
     able to connect to some servers; the following example allows connecting
     to a ``Lion'' that uses OpenSSL 0.9.8za from June 2014 (refer to INTERNAL
     VARIABLES for more on variable chains):
           wysh set tls-config-pairs-lion AT exam.ple='MinProtocol=TLSv1.1,\
               CipherString=TLSv1.2:!aNULL:!eNULL:\
                 ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:\
                 DHE-RSA-AES256-SHA:@STRENGTH'
     The OpenSSL program ciphers(1) should be referred to when creating a cus-
     tom cipher list.  Variables of interest for TLS in general are
     tls-ca-dir, tls-ca-file, tls-ca-flags, tls-ca-no-defaults,
     tls-config-file, tls-config-module, tls-config-pairs, tls-crl-dir,
     tls-crl-file, tls-rand-file as well as tls-verify.  Also see
     tls-features.  TLS is available if `+tls' is included in features.
   Character sets
     [Option] The user's locale environment is detected by looking at the
     LC_ALL environment variable.  The internal variable ttycharset will be
     set to the detected terminal character set accordingly, and will thus
     show up in the output of commands like set and varshow.  This character
     set will be targeted when trying to display data, and user input data is
     expected to be in this character set, too.
     When creating messages their character input data is classified.  7-bit
     clean text data and attachments will be classified as charset-7bit.
     8-bit data will [Option]ally be converted into members of sendcharsets
     until a character set conversion succeeds.  charset-8bit is the implied
     default last member of this list.  If no 8-bit character set is capable
     to represent input data, no message will be sent, and its text will
     optionally be saved in DEAD.  If that is not acceptable, for example in
     script environments, mime-force-sendout can be set to force sending of
     non-convertible data as `application/octet-stream' classified binary con-
     tent instead: like this receivers still have the option to inspect mes-
     sage content (for example via mime-counter-evidence).  If the [Option]al
     character set conversion is not available (features misses `,+iconv,'),
     ttycharset is the only supported character set for non 7-bit clean data,
     and it is simply assumed it can be used to exchange 8-bit messages.
     ttycharset may also be given an explicit value to send mail in a com-
     pletely ``faked'' locale environment, which can be used to generate and
     send for example 8-bit UTF-8 input data in a pure 7-bit US-ASCII
     `LC_ALL=C' environment (an example of this can be found in the section On
     sending mail, and non-interactive mode).  Due to lack of programming
     interfaces reading mail will not really work as expected in a faked envi-
     ronment: whereas ttycharset might be addressable, any output will be made
     safely printable, as via vexpr makeprint, according to the actual locale
     environment, which is not affected by ttycharset.
     Classifying 7-bit clean data as charset-7bit is a problem if the input
     character set (ttycharset) is a multibyte character set that is itself
     7-bit clean.  For example, the Japanese character set ISO-2022-JP is, but
     is capable to encode the rich set of Japanese Kanji, Hiragana and
     Katakana characters: in order to notify receivers of this character set
     the mail message must be MIME encoded so that the character set
     ISO-2022-JP can be advertised, otherwise an invalid email message would
     result!  To achieve this, the variable charset-7bit can be set to
     ISO-2022-JP.  (Today a better approach regarding email is the usage of
     UTF-8, which uses 8-bit bytes for non-US-ASCII data.)
     When replying to a message and the variable reply-in-same-charset is set,
     the character set of the message being replied to is tried first as a
     target character set (still being a subject of charsetalias filtering,
     however).  Another opportunity is sendcharsets-else-ttycharset to reflect
     the user's locale environment automatically, it will treat ttycharset as
     an implied member of (an unset) sendcharsets.
     [Option] When reading messages, their text data is converted into
     ttycharset as necessary in order to display them on the user's terminal.
     Unprintable characters and invalid byte sequences are detected and
     replaced by substitution characters.  Character set mappings for source
     character sets can be established with charsetalias, which may be handy
     to work around faulty or incomplete character set catalogues (one could
     for example add a missing LATIN1 to ISO-8859-1 mapping), or to enforce
     treatment of one character set as another one (``interpret LATIN1 as
     CP1252'').  Also see charset-unknown-8bit to deal with another hairy
     aspect of message interpretation.
     In general, if a message saying ``cannot convert from a to b'' appears,
     either some characters are not appropriate for the currently selected
     (terminal) character set, or the needed conversion is not supported by
     the system.  In the first case, it is necessary to set an appropriate
     LC_CTYPE locale and/or the variable ttycharset.  The best results are
     usually achieved when running in a UTF-8 locale on a UTF-8 capable termi-
     nal, in which case the full Unicode spectrum of characters is available.
     In this setup characters from various countries can be displayed, while
     it is still possible to use more simple character sets for sending to
     retain maximum compatibility with older mail clients.
     On the other hand the POSIX standard defines a locale-independent 7-bit
     ``portable character set'' that should be used when overall portability
     is an issue, the even more restricted subset named ``portable filename
     character set'' consists of A-Z, a-z, 0-9, period `.', underscore `_' and
     hyphen-minus `-'.
   Message states
     S-nail differentiates in between several message states; the current
     state will be reflected in the summary of headers if the attrlist of the
     configured headline allows, and Specifying messages dependent on their
     state is possible.  When operating on the system inbox, or in any other
     primary system mailbox, special actions, like the automatic moving of
     messages to the secondary mailbox MBOX, may be applied when the mailbox
     is left (also implicitly by program termination, unless the command exit
     was used) - however, because this may be irritating to users which are
     used to ``more modern'' mail-user-agents, the provided global s-nail.rc
     template sets the internal hold and keepsave variables in order to sup-
     press this behaviour.
     `new'     Message has neither been viewed nor moved to any other state.
               Such messages are retained even in the primary system mailbox.
     `unread'  Message has neither been viewed nor moved to any other state,
               but the message was present already when the mailbox has been
               opened last: Such messages are retained even in the primary
               system mailbox.
     `read'    The message has been processed by one of the following com-
               mands: ~f, ~m, ~F, ~M, copy, mbox, next, pipe, Print, print,
               top, Type, type, undelete.  The commands dp and dt will always
               try to automatically ``step'' and type the ``next'' logical
               message, and may thus mark multiple messages as read, the
               delete command will do so if the internal variable autoprint is
               set.
               Except when the exit command is used, messages that are in a
               primary system mailbox and are in `read' state when the mailbox
               is left will be saved in the secondary mailbox MBOX unless the
               internal variable hold it set.
     `deleted' The message has been processed by one of the following com-
               mands: delete, dp, dt.  Only undelete can be used to access
               such messages.
     `preserved' The message has been processed by a preserve command and it
               will be retained in its current location.
     `saved'   The message has been processed by one of the following com-
               mands: save or write.  Unless when the exit command is used,
               messages that are in a primary system mailbox and are in
               `saved' state when the mailbox is left will be deleted; they
               will be saved in the secondary mailbox MBOX when the internal
               variable keepsave is set.
     In addition to these message states, flags which otherwise have no tech-
     nical meaning in the mail system except allowing special ways of address-
     ing them when Specifying messages can be set on messages.  These flags
     are saved with messages and are thus persistent, and are portable between
     a set of widely used MUAs.
     answered  Mark messages as having been answered.
     draft     Mark messages as being a draft.
     flag      Mark messages which need special attention.
   Specifying messages
     [Only new quoting rules] COMMANDS which take Message list arguments, such
     as search, type, copy, and delete, can perform actions on a number of
     messages at once.  Specifying invalid messages, or using illegal syntax,
     will cause errors to be reported through the INTERNAL VARIABLES !, ^ERR
     and companions, as well as the command exit status ?.
     For example, `delete 1 2' deletes the messages 1 and 2, whereas `delete
     1-5' will delete the messages 1 through 5.  In sorted or threaded mode
     (see the sort command), `delete 1-5' will delete the messages that are
     located between (and including) messages 1 through 5 in the
     sorted/threaded order, as shown in the headers summary.
     Errors can for example be ^ERR-BADMSG when requesting an invalid message,
     ^ERR-NOMSG if no applicable message can be found, ^ERR-CANCELED for miss-
     ing informational data (mostly thread-related).  ^ERR-INVAL for invalid
     syntax as well as ^ERR-IO for input/output errors can happen.  The fol-
     lowing special message names exist:
     .     The current message, the so-called ``dot''.
     ;     The message that was previously the current message; needs to be
           quoted.
     ,     The parent message of the current message, that is the message with
           the Message-ID given in the `In-Reply-To:' field or the last entry
           of the `References:' field of the current message.
     -     The previous undeleted message, or the previous deleted message for
           the undelete command; In sorted or `thread'ed mode, the previous
           such message in the according order.
     +     The next undeleted message, or the next deleted message for the
           undelete command; In sorted or `thread'ed mode, the next such mes-
           sage in the according order.
     ^     The first undeleted message, or the first deleted message for the
           undelete command; In sorted or `thread'ed mode, the first such mes-
           sage in the according order.
     $     The last message; In sorted or `thread'ed mode, the last such mes-
           sage in the according order.  Needs to be quoted.
     &x    In `thread'ed sort mode, selects the message addressed with x,
           where x is any other message specification, and all messages from
           the thread that begins at it.  Otherwise it is identical to x.  If
           x is omitted, the thread beginning with the current message is
           selected.
     *     All messages.
     `     All messages that were included in the Message list arguments of
           the previous command; needs to be quoted.  (A convenient way to
           read all new messages is to select them via `from :n', as below,
           and then to read them in order with the default command -- next --
           simply by successively typing ``'; for this to work showlast must
           be set.)
     x-y   An inclusive range of message numbers.  Selectors that may also be
           used as endpoints include any of .;-+^$.
     address
           A case-insensitive ``any substring matches'' search against the
           `From:' header, which will match addresses (too) even if showname
           is set (and POSIX says ``any address as shown in a header summary
           shall be matchable in this form''); However, if the allnet variable
           is set, only the local part of the address is evaluated for the
           comparison, not ignoring case, and the setting of showname is com-
           pletely ignored.  For finer control and match boundaries use the
           `@' search expression.
     /string
           All messages that contain string in the subject field (case ignored
           according to locale).  See also the searchheaders variable.  If
           string is empty, the string from the previous specification of that
           type is used again.
     [@name-list]@expr
           All messages that contain the given case-insensitive search
           expression;  If the [Option]al regular expression support is avail-
           able expr will be interpreted as (an extended) one if any of the
           magic regular expression characters is seen.  If the optional
           @name-list part is missing the search is restricted to the subject
           field body, but otherwise name-list specifies a comma-separated
           list of header fields to search, for example
                 '@to,from,cc@Someone i ought to know'
           In order to search for a string that includes a `@' (commercial at)
           character the name-list is effectively non-optional, but may be
           given as the empty string.  Also, specifying an empty search
           expression will effectively test for existence of the given header
           fields.  Some special header fields may be abbreviated: `f', `t',
           `c', `b' and `s' will match `From', `To', `Cc', `Bcc' and
           `Subject', respectively and case-insensitively.  [Option]ally, and
           just like expr, name-list will be interpreted as (an extended) reg-
           ular expression if any of the magic regular expression characters
           is seen.
           The special names `header' or `<' can be used to search in (all of)
           the header(s) of the message, and the special names `body' or `>'
           and `text' or `=' will perform full text searches - whereas the
           former searches only the body, the latter also searches the message
           header ([v15 behaviour may differ] this mode yet brute force
           searches over the entire decoded content of messages, including
           administrativa strings).
           This specification performs full text comparison, but even with
           regular expression support it is almost impossible to write a
           search expression that safely matches only a specific address
           domain.  To request that the body content of the header is treated
           as a list of addresses, and to strip those down to the plain email
           address which the search expression is to be matched against, pre-
           fix the effective name-list with a tilde `~':
                 '@~f,c@@a\.safe\.domain\.match$'
     :c    All messages of state or with matching condition `c', where `c' is
           one or multiple of the following colon modifiers:
           a   answered messages (cf. the variable markanswered).
           d   `deleted' messages (for the undelete and from commands only).
           f   flagged messages.
           L   Messages with receivers that match mlsubscribed addresses.
           l   Messages with receivers that match mlisted addresses.
           n   `new' messages.
           o   Old messages (any not in state `read' or `new').
           r   `read' messages.
           S   [Option] Messages with unsure spam classification (see Handling
               spam).
           s   [Option] Messages classified as spam.
           t   Messages marked as draft.
           u   `unread' messages.
     [Option] IMAP-style SEARCH expressions may also be used.  These consist
     of keywords and criterions, and because Message list arguments are split
     into tokens according to Shell-style argument quoting it is necessary to
     quote the entire IMAP search expression in order to ensure that it
     remains a single token.  This addressing mode is available with all types
     of mailbox folders; S-nail will perform the search locally as necessary.
     Strings must be enclosed by double quotation marks `"' in their entirety
     if they contain whitespace or parentheses; within the quotes, only
     reverse solidus `\' is recognized as an escape character.  All string
     searches are case-insensitive.  When the description indicates that the
     ``envelope'' representation of an address field is used, this means that
     the search string is checked against both a list constructed as
           '("name" "source" "local-part" "domain-part")'
     for each address, and the addresses without real names from the respec-
     tive header field.  These search expressions can be nested using paren-
     theses, see below for examples.
     (criterion)
           All messages that satisfy the given criterion.
     (criterion1 criterion2 ... criterionN)
           All messages that satisfy all of the given criteria.
     (or criterion1 criterion2)
           All messages that satisfy either criterion1 or criterion2, or both.
           To connect more than two criteria using `or' specifications have to
           be nested using additional parentheses, as with `(or a (or b c))',
           since `(or a b c)' really means `((a or b) and c)'.  For a simple
           `or' operation of independent criteria on the lowest nesting level,
           it is possible to achieve similar effects by using three separate
           criteria, as with `(a) (b) (c)'.
     (not criterion)
           All messages that do not satisfy criterion.
     (bcc "string")
           All messages that contain string in the envelope representation of
           the `Bcc:' field.
     (cc "string")
           All messages that contain string in the envelope representation of
           the `Cc:' field.
     (from "string")
           All messages that contain string in the envelope representation of
           the `From:' field.
     (subject "string")
           All messages that contain string in the `Subject:' field.
     (to "string")
           All messages that contain string in the envelope representation of
           the `To:' field.
     (header name "string")
           All messages that contain string in the specified `Name:' field.
     (body "string")
           All messages that contain string in their body.
     (text "string")
           All messages that contain string in their header or body.
     (larger size)
           All messages that are larger than size (in bytes).
     (smaller size)
           All messages that are smaller than size (in bytes).
     (before date)
           All messages that were received before date, which must be in the
           form `d[d]-mon-yyyy', where `d' denotes the day of the month as one
           or two digits, `mon' is the name of the month - one of `Jan Feb Mar
           Apr May Jun Jul Aug Sep Oct Nov Dec', and `yyyy' is the year as
           four digits, for example `28-Dec-2012'.
     (on date)
           All messages that were received on the specified date.
     (since date)
           All messages that were received since the specified date.
     (sentbefore date)
           All messages that were sent on the specified date.
     (senton date)
           All messages that were sent on the specified date.
     (sentsince date)
           All messages that were sent since the specified date.
     ()    The same criterion as for the previous search.  This specification
           cannot be used as part of another criterion.  If the previous com-
           mand line contained more than one independent criterion then the
           last of those criteria is used.
   On terminal control and line editor
     [Option] Terminal control through one of the standard UNIX libraries,
     Termcap Access Library (libtermcap, -ltermcap) or Terminal Information
     Library (libterminfo, -lterminfo), may be available.  For the TERMinal
     defined in the environment interactive usage aspects, for example
     Coloured display, and insight of cursor and function keys for the Mailx-
     Line-Editor (MLE), will be enhanced or enabled.  Library interaction can
     be disabled on a per-invocation basis via termcap-disable, whereas the
     internal variable termcap is always used as a preferred source of termi-
     nal capabilities.  (For a usage example see the FAQ entry Not
     "defunctional", but the editor key does not work.)
     [Option] The built-in Mailx-Line-Editor (MLE) should work in all environ-
     ments which comply to the ISO C standard ISO/IEC 9899/AMD1:1995
     (``ISO C90, Amendment 1''), and will support wide glyphs if possible (the
     necessary functionality had been removed from ISO C, but was included in
     X/Open Portability Guide Issue 4 (``XPG4'')).  Usage of a line editor in
     interactive mode can be prevented by setting line-editor-disable.  Espe-
     cially if the [Option]al terminal control support is missing setting
     entries in termcap will help shall the MLE misbehave, see there for more.
     The MLE can support a little bit of colour.
     [Option] If the history feature is available then input from line editor
     prompts will be saved in a history list that can be searched in and be
     expanded from.  Such saving can be prevented by prefixing input with any
     amount of whitespace.  Aspects of history, like allowed content and maxi-
     mum size, as well as whether history shall be saved persistently, can be
     configured with the internal variables history-file, history-gabby,
     history-gabby-persist and history-size.  There also exists the macro hook
     on-history-addition which can be used to apply finer control on what
     enters history.
     The MLE supports a set of editing and control commands.  By default (as)
     many (as possible) of these will be assigned to a set of single-letter
     control codes, which should work on any terminal (and can be generated by
     holding the ``control'' key while pressing the key of desire, for example
     `control-D').  If the [Option]al bind command is available then the MLE
     commands can also be accessed freely by assigning the command name, which
     is shown in parenthesis in the list below, to any desired key-sequence,
     and the MLE will instead and also use bind to establish its built-in key
     bindings (more of them if the [Option]al terminal control is available),
     an action which can then be suppressed completely by setting
     line-editor-no-defaults.  Shell-style argument quoting notation is used
     in the following:
     `\cA'  Go to the start of the line (mle-go-home).
     `\cB'  Move the cursor backward one character (mle-go-bwd).
     `\cC'  raise(3) `SIGINT' (mle-raise-int).
     `\cD'  Forward delete the character under the cursor; quits S-nail if
            used on the empty line unless the internal variable ignoreeof is
            set (mle-del-fwd).
     `\cE'  Go to the end of the line (mle-go-end).
     `\cF'  Move the cursor forward one character (mle-go-fwd).
     `\cG'  Cancel current operation, full reset.  If there is an active his-
            tory search or tabulator expansion then this command will first
            reset that, reverting to the former line content; thus a second
            reset is needed for a full reset in this case (mle-reset).
     `\cH'  Backspace: backward delete one character (mle-del-bwd).
     `\cI'  [Only new quoting rules] Horizontal tabulator: try to expand the
            word before the cursor, supporting the usual Filename
            transformations (mle-complete; this is affected by
            mle-quote-rndtrip and line-editor-cpl-word-breaks).
     `\cJ'  Newline: commit the current line (mle-commit).
     `\cK'  Cut all characters from the cursor to the end of the line
            (mle-snarf-end).
     `\cL'  Repaint the line (mle-repaint).
     `\cN'  [Option] Go to the next history entry (mle-hist-fwd).
     `\cO'  ([Option]ally context-dependent) Invokes the command dt.
     `\cP'  [Option] Go to the previous history entry (mle-hist-bwd).
     `\cQ'  Toggle roundtrip mode shell quotes, where produced, on and off
            (mle-quote-rndtrip).  This setting is temporary, and will be for-
            gotten once the command line is committed; also see shcodec.
     `\cR'  [Option] Complete the current line from (the remaining) older his-
            tory entries (mle-hist-srch-bwd).
     `\cS'  [Option] Complete the current line from (the remaining) newer his-
            tory entries (mle-hist-srch-fwd).
     `\cT'  Paste the snarf buffer (mle-paste).
     `\cU'  The same as `\cA' followed by `\cK' (mle-snarf-line).
     `\cV'  Prompts for a Unicode character (hexadecimal number without pre-
            fix, see vexpr) to be inserted (mle-prompt-char).  Note this com-
            mand needs to be assigned to a single-letter control code in order
            to become recognized and executed during input of a key-sequence
            (only three single-letter control codes can be used for that
            shortcut purpose); this control code is then special-treated and
            thus cannot be part of any other sequence (because it will trigger
            the mle-prompt-char function immediately).
     `\cW'  Cut the characters from the one preceding the cursor to the pre-
            ceding word boundary (mle-snarf-word-bwd).
     `\cX'  Move the cursor forward one word boundary (mle-go-word-fwd).
     `\cY'  Move the cursor backward one word boundary (mle-go-word-bwd).
     `\cZ'  raise(3) `SIGTSTP' (mle-raise-tstp).
     `\c['  Escape: reset a possibly used multibyte character input state
            machine and [Option]ally a lingering, incomplete key binding
            (mle-cancel).  This command needs to be assigned to a single-let-
            ter control code in order to become recognized and executed during
            input of a key-sequence (only three single-letter control codes
            can be used for that shortcut purpose).  This control code may
            also be part of a multi-byte sequence, but if a sequence is active
            and the very control code is currently also an expected input,
            then the active sequence takes precedence and will consume the
            control code.
     `\c\'  ([Option]ally context-dependent) Invokes the command `z+'.
     `\c]'  ([Option]ally context-dependent) Invokes the command `z$'.
     `\c^'  ([Option]ally context-dependent) Invokes the command `z0'.
     `\c_'  Cut the characters from the one after the cursor to the succeeding
            word boundary (mle-snarf-word-fwd).
     `\c?'  Backspace: mle-del-bwd.
     -      mle-bell: ring the audible bell.
     -      [Option] mle-clear-screen: move the cursor home and clear the
            screen.
     -      mle-fullreset: different to mle-reset this will immediately reset
            a possibly active search etc.
     -      mle-go-screen-bwd: move the cursor backward one screen width.
     -      mle-go-screen-fwd: move the cursor forward one screen width.
     -      mle-raise-quit: raise(3) `SIGQUIT'.
   Coloured display
     [Option] Colours and font attributes through ANSI a.k.a. ISO 6429 SGR
     (select graphic rendition) escape sequences are optionally supported.
     Usage of colours and font attributes solely depends upon the capability
     of the detected terminal type (TERM), and as fine-tuned through termcap.
     Colours and font attributes can be managed with the multiplexer command
     colour, and uncolour removes the given mappings.  Setting colour-disable
     suppresses usage of colour and font attribute sequences, while leaving
     established mappings unchanged.
     Whether actually applicable colour and font attribute sequences should
     also be generated when output is going to be paged through the external
     PAGER (also see crt) depends upon the setting of colour-pager, because
     pagers usually need to be configured in order to support ISO escape
     sequences.  Knowledge of some widely used pagers is however built-in, and
     in a clean environment it is often enough to simply set colour-pager;
     please refer to that variable for more on this topic.
     It might make sense to conditionalize colour setup on interactive mode
     via if (`terminal' indeed means ``interactive''):
           if terminal && "$features" =% ,+colour,
             colour iso view-msginfo ft=bold,fg=green
             colour iso view-header ft=bold,fg=red (from|subject) # regex
             colour iso view-header fg=red
             uncolour iso view-header from,subject
             colour iso view-header ft=bold,fg=magenta,bg=cyan
             colour 256 view-header ft=bold,fg=208,bg=230 "subject,from"
             colour mono view-header ft=bold
             colour mono view-header ft=bold,ft=reverse subject,from
           endif
   Handling spam
     [Option] S-nail can make use of several spam interfaces for the purpose
     of identification of, and, in general, dealing with spam messages.  A
     precondition of most commands in order to function is that the
     spam-interface variable is set to one of the supported interfaces.
     Specifying messages that have been identified as spam is possible via
     their (volatile) `is-spam' state by using the `:s' and `:S' specifica-
     tions, and their attrlist entries will be used when displaying the
     headline in the summary of headers.
     o   spamrate rates the given messages and sets their `is-spam' flag
         accordingly.  If the spam interface offers spam scores these can be
         shown in headline by using the format `%$'.
     o   spamham, spamspam and spamforget will interact with the Bayesian fil-
         ter of the chosen interface and learn the given messages as ``ham''
         or ``spam'', respectively; the last command can be used to cause
         ``unlearning'' of messages; it adheres to their current `is-spam'
         state and thus reverts previous teachings.
     o   spamclear and spamset will simply set and clear, respectively, the
         mentioned volatile `is-spam' message flag, without any interface
         interaction.
     The spamassassin(1) based spam-interface `spamc' requires a running
     instance of the spamd(1) server in order to function, started with the
     option --allow-tell shall Bayesian filter learning be possible.
           $ spamd -i localhost:2142 -i /tmp/.spamsock -d [-L] [-l]
           $ spamd --listen=localhost:2142 --listen=/tmp/.spamsock \
               --daemonize [--local] [--allow-tell]
     Thereafter S-nail can make use of these interfaces:
           $ s-nail -Sspam-interface=spamc -Sspam-maxsize=500000 \
               -Sspamc-command=/usr/local/bin/spamc \
               -Sspamc-arguments="-U /tmp/.spamsock" -Sspamc-user=
           or
           $ s-nail -Sspam-interface=spamc -Sspam-maxsize=500000 \
               -Sspamc-command=/usr/local/bin/spamc \
               -Sspamc-arguments="-d localhost -p 2142" -Sspamc-user=
     Using the generic filter approach allows usage of programs like
     bogofilter(1).  Here is an example, requiring it to be accessible via
     PATH:
           $ s-nail -Sspam-interface=filter -Sspam-maxsize=500000 \
               -Sspamfilter-ham="bogofilter -n" \
               -Sspamfilter-noham="bogofilter -N" \
               -Sspamfilter-nospam="bogofilter -S" \
               -Sspamfilter-rate="bogofilter -TTu 2>/dev/null" \
               -Sspamfilter-spam="bogofilter -s" \
               -Sspamfilter-rate-scanscore="1;^(.+)$"
     Because messages must exist on local storage in order to be scored (or
     used for Bayesian filter training), it is possibly a good idea to perform
     the local spam check last.  Spam can be checked automatically when open-
     ing specific folders by setting a specialized form of the internal vari-
     able folder-hook.
           define spamdelhook {
             # Server side DCC
             spamset (header x-dcc-brand-metrics "bulk")
             # Server-side spamassassin(1)
             spamset (header x-spam-flag "YES")
             del :s # TODO we HAVE to be able to do `spamrate :u ! :sS'
             move :S +maybe-spam
             spamrate :u
             del :s
             move :S +maybe-spam
           }
           set folder-hook-SOMEFOLDER=spamdelhook
     See also the documentation for the variables spam-interface,
     spam-maxsize, spamc-command, spamc-arguments, spamc-user, spamfilter-ham,
     spamfilter-noham, spamfilter-nospam, spamfilter-rate and
     spamfilter-rate-scanscore.
COMMANDS
     S-nail reads input in lines.  An unquoted reverse solidus `\' at the end
     of a command line ``escapes'' the newline character: it is discarded and
     the next line of input is used as a follow-up line, with all leading
     whitespace removed; once an entire line is completed, the whitespace
     characters space, tabulator, newline as well as those defined by the
     variable ifs are removed from the beginning and end.  Placing any white-
     space characters at the beginning of a line will prevent a possible addi-
     tion of the command line to the [Option]al history.
     The beginning of such input lines is then scanned for the name of a known
     command: command names may be abbreviated, in which case the first com-
     mand that matches the given prefix will be used.  Command modifiers may
     prefix a command in order to modify its behaviour.  A name may also be a
     commandalias, which will become expanded until no more expansion is pos-
     sible.  Once the command that shall be executed is known, the remains of
     the input line will be interpreted according to command-specific rules,
     documented in the following.
     This behaviour is different to the sh(1)ell, which is a programming lan-
     guage with syntactic elements of clearly defined semantics, and therefore
     capable to sequentially expand and evaluate individual elements of a
     line.  `? set one=value two=$one' for example will never possibly assign
     value to one, because the variable assignment is performed no sooner but
     by the command (set), long after the expansion happened.
     A list of all commands in lookup order is dumped by the command list.
     [Option]ally the command help (or ?), when given an argument, will show a
     documentation string for the command matching the expanded argument, as
     in `?t', which should be a shorthand of `?type'; with these documentation
     strings both commands support a more verbose listing mode which includes
     the argument type of the command and other information which applies; a
     handy suggestion might thus be:
           ? define __xv {
             # Before v15: need to enable sh(1)ell-style on _entire_ line!
             localopts yes;wysh set verbose;ignerr eval "${@}";return ${?}
           }
           ? commandalias xv '\call __xv'
           ? xv help set
   Command modifiers
     Commands may be prefixed by none to multiple command modifiers.  Some
     command modifiers can be used with a restricted set of commands only, the
     verbose version of list will ([Option]ally) show which modifiers apply.
     o   The modifier reverse solidus \, to be placed first, prevents
         commandalias expansions on the remains of the line, for example
         `\echo' will always evaluate the command echo, even if an (com-
         mand)alias of the same name exists.  commandalias content may itself
         contain further command modifiers, including an initial reverse
         solidus to prevent further expansions.
     o   The modifier ignerr indicates that any error generated by the follow-
         ing command should be ignored by the state machine and not cause a
         program exit with enabled errexit or for the standardized exit cases
         in posix mode.  ?, one of the INTERNAL VARIABLES, will be set to the
         real exit status of the command regardless.
     o   local will alter the called command to apply changes only temporar-
         ily, local to block-scope, and can thus only be used inside of a
         defined macro or an account definition.  Specifying it implies the
         modifier wysh.  Local variables will not be inherited by macros
         deeper in the call chain, and all local settings will be garbage col-
         lected once the local scope is left.  To record and unroll changes in
         the global scope use the command localopts.
     o   scope does yet not implement any functionality.
     o   u does yet not implement any functionality.
     o   Some commands support the vput modifier: if used, they expect the
         name of a variable, which can itself be a variable, i.e., shell
         expansion is applied, as their first argument, and will place their
         computation result in it instead of the default location (it is usu-
         ally written to standard output).
         The given name will be tested for being a valid sh(1) variable name,
         and may therefore only consist of upper- and lowercase characters,
         digits, and the underscore; the hyphen-minus may be used as a non-
         portable extension; digits may not be used as first, hyphen-minus may
         not be used as last characters.  In addition the name may either not
         be one of the known INTERNAL VARIABLES, or must otherwise refer to a
         writable (non-boolean) value variable.  The actual put operation may
         fail nonetheless, for example if the variable expects a number argu-
         ment only a number will be accepted.  Any error during these opera-
         tions causes the command as such to fail, and the error number ! will
         be set to ^ERR-NOTSUP, the exit status ? should be set to `-1', but
         some commands deviate from the latter, which is documented.
     o   Last, but not least, the modifier wysh can be used for some old and
         established commands to choose the new Shell-style argument quoting
         rules over the traditional Old-style argument quoting.  This modifier
         is implied if v15-compat is set to a non-empty value.
   Old-style argument quoting
     [v15 behaviour may differ] This section documents the traditional and
     POSIX standardized style of quoting non-message list arguments to com-
     mands which expect this type of arguments: whereas still used by the
     majority of such commands, the new Shell-style argument quoting may be
     available even for those via wysh, one of the Command modifiers.  None-
     theless care must be taken, because only new commands have been designed
     with all the capabilities of the new quoting rules in mind, which can,
     for example generate control characters.
           o   An argument can be enclosed between paired double-quotes
               `"argument"' or single-quotes `'argument''; any whitespace,
               shell word expansion, or reverse solidus characters (except as
               described next) within the quotes are treated literally as part
               of the argument.  A double-quote will be treated literally
               within single-quotes and vice versa.  Inside such a quoted
               string the actually used quote character can be used nonethe-
               less by escaping it with a reverse solidus `\', as in
               `"y\"ou"'.
           o   An argument that is not enclosed in quotes, as above, can usu-
               ally still contain space characters if those spaces are reverse
               solidus escaped, as in `you\ are'.
           o   A reverse solidus outside of the enclosing quotes is discarded
               and the following character is treated literally as part of the
               argument.
   Shell-style argument quoting
     sh(1)ell-style, and therefore POSIX standardized, argument parsing and
     quoting rules are used by most commands.  [v15 behaviour may differ] Most
     new commands only support these new rules and are flagged [Only new quot-
     ing rules], some elder ones can use them with the command modifier wysh;
     in the future only this type of argument quoting will remain.
     A command line is parsed from left to right and an input token is com-
     pleted whenever an unquoted, otherwise ignored, metacharacter is seen.
     Metacharacters are vertical bar |, ampersand &, semicolon ;, as well as
     all characters from the variable ifs, and / or space, tabulator, newline.
     The additional metacharacters left and right parenthesis (, ) and less-
     than and greater-than signs <, > that the sh(1) supports are not used,
     and are treated as ordinary characters: for one these characters are a
     vivid part of email addresses, and it seems highly unlikely that their
     function will become meaningful to S-nail.
           Compatibility note: [v15 behaviour may  differ]  Please  note  that
           even  many  new-style commands do not yet honour ifs to parse their
           arguments: whereas the sh(1)ell is a language with  syntactic  ele-
           ments  of  clearly  defined  semantics,  S-nail parses entire input
           lines and decides on a per-command base what to do with the rest of
           the line.  This also means that whenever an unknown command is seen
           all that S-nail can do is cancellation of  the  processing  of  the
           remains of the line.
           It also often depends on an actual subcommand of a multiplexer com-
           mand how the rest of the line should be treated, and until  v15  we
           are  not  capable  to  perform  this  deep inspection of arguments.
           Nonetheless, at least the following commands which work with  posi-
           tional  parameters fully support ifs for an almost shell-compatible
           field splitting: call, call_if, read, vpospar, xcall.
     Any unquoted number sign `#' at the beginning of a new token starts a
     comment that extends to the end of the line, and therefore ends argument
     processing.  An unquoted dollar sign `$' will cause variable expansion of
     the given name, which must be a valid sh(1)ell-style variable name (see
     vput): INTERNAL VARIABLES as well as ENVIRONMENT (shell) variables can be
     accessed through this mechanism, brace enclosing the name is supported
     (i.e., to subdivide a token).
     Whereas the metacharacters space, tabulator, newline only complete an
     input token, vertical bar |, ampersand & and semicolon ; also act as con-
     trol operators and perform control functions.  For now supported is semi-
     colon ;, which terminates a single command, therefore sequencing the com-
     mand line and making the remainder of the line a subject to reevaluation.
     With sequencing, multiple command argument types and quoting rules may
     therefore apply to a single line, which can become problematic before
     v15: e.g., the first of the following will cause surprising results.
           ? echo one; set verbose; echo verbose=$verbose.
           ? echo one; wysh set verbose; echo verbose=$verbose.
     Quoting is a mechanism that will remove the special meaning of metachar-
     acters and reserved words, and will prevent expansion.  There are four
     quoting mechanisms: the escape character, single-quotes, double-quotes
     and dollar-single-quotes:
           o   The literal value of any character can be preserved by preced-
               ing it with the escape character reverse solidus `\'.
           o   Arguments which are enclosed in `'single-quotes'' retain their
               literal value.  A single-quote cannot occur within single-
               quotes.
           o   The literal value of all characters enclosed in `"double-
               quotes"' is retained, with the exception of dollar sign `$',
               which will cause variable expansion, as above, backquote (grave
               accent) ``', (which not yet means anything special), reverse
               solidus `\', which will escape any of the characters dollar
               sign `$' (to prevent variable expansion), backquote (grave
               accent) ``', double-quote `"' (to prevent ending the quote) and
               reverse solidus `\' (to prevent escaping, i.e., to embed a
               reverse solidus character as-is), but has no special meaning
               otherwise.
           o   Arguments enclosed in `$'dollar-single-quotes'' extend normal
               single quotes in that reverse solidus escape sequences are
               expanded as follows:
               `\a'    bell control character (ASCII and ISO-10646 BEL).
               `\b'    backspace control character (ASCII and ISO-10646 BS).
               `\E'    escape control character (ASCII and ISO-10646 ESC).
               `\e'    the same.
               `\f'    form feed control character (ASCII and ISO-10646 FF).
               `\n'    line feed control character (ASCII and ISO-10646 LF).
               `\r'    carriage return control character (ASCII and ISO-10646
                       CR).
               `\t'    horizontal tabulator control character (ASCII and
                       ISO-10646 HT).
               `\v'    vertical tabulator control character (ASCII and
                       ISO-10646 VT).
               `\\'    emits a reverse solidus character.
               `\''    single quote.
               `\"'    double quote (escaping is optional).
               `\NNN'  eight-bit byte with the octal value `NNN' (one to three
                       octal digits), optionally prefixed by an additional
                       `0'.  A 0 byte will suppress further output for the
                       quoted argument.
               `\xHH'  eight-bit byte with the hexadecimal value `HH' (one or
                       two hexadecimal characters, no prefix, see vexpr).  A 0
                       byte will suppress further output for the quoted argu-
                       ment.
               `\UHHHHHHHH'
                       the Unicode / ISO-10646 character with the hexadecimal
                       codepoint value `HHHHHHHH' (one to eight hexadecimal
                       characters) -- note that Unicode defines the maximum
                       codepoint ever to be supported as `0x10FFFF' (in planes
                       of `0xFFFF' characters each).  This escape is only sup-
                       ported in locales that support Unicode (see Character
                       sets), in other cases the sequence will remain unex-
                       panded unless the given code point is ASCII compatible
                       or (if the [Option]al character set conversion is
                       available) can be represented in the current locale.
                       The character NUL will suppress further output for the
                       quoted argument.
               `\uHHHH'
                       Identical to `\UHHHHHHHH' except it takes only one to
                       four hexadecimal characters.
               `\cX'   Emits the non-printable (ASCII and compatible) C0 con-
                       trol codes 0 (NUL) to 31 (US), and 127 (DEL).  Print-
                       able representations of ASCII control codes can be cre-
                       ated by mapping them to a different, visible part of
                       the ASCII character set.  Adding the number 64 achieves
                       this for the codes 0 to 31, here 7 (BEL): `7 + 64 = 71
                       = G'.  The real operation is a bitwise logical XOR with
                       64 (bit 7 set, see vexpr), thus also covering code 127
                       (DEL), which is mapped to 63 (question mark):
                       `? vexpr ^ 127 64'.
                       Whereas historically circumflex notation has often been
                       used for visualization purposes of control codes, as in
                       `^G', the reverse solidus notation has been standard-
                       ized: `\cG'.  Some control codes also have standardized
                       (ISO-10646, ISO C) aliases, as shown above (`\a', `\n',
                       `\t' etc) : whenever such an alias exists it will be
                       used for display purposes.  The control code NUL
                       (`\c@', a non-standard extension) will suppress further
                       output for the remains of the token (which may extend
                       beyond the current quote), or, depending on the con-
                       text, the remains of all arguments for the current com-
                       mand.
               `\$NAME'
                       Non-standard extension: expand the given variable name,
                       as above.  Brace enclosing the name is supported.
               `\`{command}'
                       Not yet supported, just to raise awareness: Non-stan-
                       dard extension.
     Caveats:
           ? echo 'Quotes '${HOME}' and 'tokens" differ!"# no comment
           ? echo Quotes ${HOME} and tokens differ! # comment
           ? echo Don"'"t you worry$'\x21' The sun shines on us. $'\u263A'
   Message list arguments
     Many commands operate on message list specifications, as documented in
     Specifying messages.  The argument input is first split into individual
     tokens via Shell-style argument quoting, which are then interpreted as
     the mentioned specifications.  If no explicit message list has been spec-
     ified, many commands will search for and use the next message forward
     that satisfies the commands' requirements, and if there are no messages
     forward of the current message, the search proceeds backwards; if there
     are no good messages at all to be found, an error message is shown and
     the command is aborted.  The verbose output of the command list will
     indicate whether a command searches for a default message, or not.
   Raw data arguments for codec commands
     A special set of commands, which all have the string ``codec'' in their
     name, like addrcodec, shcodec, urlcodec, take raw string data as input,
     which means that the content of the command input line is passed com-
     pletely unexpanded and otherwise unchanged: like this the effect of the
     actual codec is visible without any noise of possible shell quoting rules
     etc., i.e., the user can input one-to-one the desired or questionable
     data.  To gain a level of expansion, the entire command line can be
     evaluated first, for example
           ? vput shcodec res encode /usr/Schones Wetter/heute.txt
           ? echo $res
           $'/usr/Sch\u00F6nes Wetter/heute.txt'
           ? shcodec d $res
           $'/usr/Sch\u00F6nes Wetter/heute.txt'
           ? eval shcodec d $res
           /usr/Schones Wetter/heute.txt
   Filename transformations
     Filenames, where expected, and unless documented otherwise, are subse-
     quently subject to the following filename transformations, in sequence:
           o   If the given name is a registered shortcut, it will be replaced
               with the expanded shortcut.  This step is mostly taken for
               folders only.
           o   The filename is matched against the following patterns or
               strings.  But for plus +file folder expansion this step is
               mostly taken for folders only.
               #      (Number sign) is expanded to the previous file.
               %      (Percent sign) is replaced by the invoking user's pri-
                      mary system mailbox, which either is the (itself expand-
                      able) inbox if that is set, the standardized absolute
                      pathname indicated by MAIL if that is set, or a built-in
                      compile-time default otherwise.  When opening a folder
                      the used name is actively checked for being a primary
                      mailbox, first against inbox, then against MAIL.
               %user  Expands to the primary system mailbox of user (and never
                      the value of inbox, regardless of its actual setting).
               &      (Ampersand) is replaced with the invoking user's sec-
                      ondary mailbox, the MBOX.
               +file  Refers to a file in the folder directory (if that vari-
                      able is set).
               %:filespec Expands to the same value as filespec, but has spe-
                      cial meaning when used with, for example, the command
                      folder: the file will be treated as a primary system
                      mailbox by, among others, the mbox and save commands,
                      meaning that messages that have been read in the current
                      session will be moved to the MBOX mailbox instead of
                      simply being flagged as read.
           o   Meta expansions may be applied to the resulting filename, as
               allowed by the operation and applicable to the resulting access
               protocol (also see On URL syntax and credential lookup).  For
               the file-protocol, a leading tilde `~' character will be
               replaced by the expansion of HOME, except when followed by a
               valid user name, in which case the home directory of the given
               user is used instead.
               A shell expansion as if specified in double-quotes (see
               Shell-style argument quoting) may be applied, so that any
               occurrence of `$VARIABLE' (or `${VARIABLE}') will be replaced
               by the expansion of the variable, if possible; INTERNAL
               VARIABLES as well as ENVIRONMENT (shell) variables can be
               accessed through this mechanism.
               Shell pathname wildcard pattern expansions (glob(7)) may be
               applied as documented.  If the fully expanded filename results
               in multiple pathnames and the command is expecting only one
               file, an error results.
               In interactive context, in order to allow simple value accep-
               tance (via ``ENTER''), arguments will usually be displayed in a
               properly quoted form, so a file `diet\ is \curd.txt' may be
               displayed as `'diet\ is \curd.txt''.
   Commands
     The following commands are available:
     !     Executes the SHELL command which follows, replacing unescaped
           exclamation marks with the previously executed command if the
           internal variable bang is set.  This command supports vput as docu-
           mented in Command modifiers, and manages the error number !.  A 0
           or positive exit status ? reflects the exit status of the command,
           negative ones that an error happened before the command was exe-
           cuted, or that the program did not exit cleanly, but maybe due to a
           signal: the error number is ^ERR-CHILD, then.
           In conjunction with the vput modifier the following special cases
           exist: a negative exit status occurs if the collected data could
           not be stored in the given variable, which is a ^ERR-NOTSUP error
           that should otherwise not occur.  ^ERR-CANCELED indicates that no
           temporary file could be created to collect the command output at
           first glance.  In case of catchable out-of-memory situations
           ^ERR-NOMEM will occur and S-nail will try to store the empty
           string, just like with all other detected error conditions.
     #     The comment-command causes the entire line to be ignored.  Note:
           this really is a normal command which' purpose is to discard its
           arguments, not a ``comment-start'' indicating special character,
           which means that for example trailing comments on a line are not
           possible (except for commands which use Shell-style argument
           quoting).
     +     Goes to the next message in sequence and types it (like ``ENTER'').
     -     Display the preceding message, or the n'th previous message if
           given a numeric argument n.
     =     Shows the message number of the current message (the ``dot'') when
           used without arguments, that of the given list otherwise.  Output
           numbers will be separated from each other with the first character
           of ifs, and followed by the first character of if-ws, if that is
           not empty and not identical to the first.  If that results in no
           separation at all a space character is used.  This command supports
           vput (see Command modifiers), and manages the error number !.
     ?     [Option] Show a brief summary of commands.  [Option] Given an argu-
           ment a synopsis for the command in question is shown instead; com-
           mands can be abbreviated in general and this command can be used to
           see the full expansion of an abbreviation including the synopsis,
           try, for example `?h', `?hel' and `?help' and see how the output
           changes.  To avoid that aliases are resolved the modifier \ can be
           prepended to the argument, but note it must be quoted.  This mode
           also supports a more verbose output, which will provide the infor-
           mation documented for list.
     |     A synonym for the pipe command.
     account, unaccount
           (ac, una) Creates, selects or lists (an) account(s).  Accounts are
           special incarnations of defined macros and group commands and vari-
           able settings which together usually arrange the environment for
           the purpose of creating an email account.  Different to normal
           macros settings which are covered by localopts - here by default
           enabled! - will not be reverted before the account is changed
           again.  The special account `null' (case-insensitive) always
           exists, and all but it can be deleted by the latter command, and in
           one operation with the special name `*'.  Also for all but it a
           possibly set on-account-cleanup hook is called once they are left,
           also for program exit.
           Without arguments a listing of all defined accounts is shown.  With
           one argument the given account is activated: the system inbox of
           that account will be activated (as via folder), a possibly
           installed folder-hook will be run, and the internal variable
           account will be updated.  The two argument form behaves identical
           to defining a macro as via define.  Important settings for accounts
           include folder, from, hostname, inbox, mta, password and user (On
           URL syntax and credential lookup), as well as things like
           tls-config-pairs (Encrypted network communication), and protocol
           specifics like imap-auth, pop3-auth, smtp-auth.
                 account myisp {
                   set folder=~/mail inbox=+syste.mbox record=+sent.mbox
                   set from='(My Name) myname AT myisp.example'
                   set mta=smtp://mylogin AT smtp.example
                 }
     addrcodec
           Perform email address codec transformations on raw-data argument,
           rather according to email standards (RFC 5322; [v15 behaviour may
           differ] will furtherly improve).  Supports vput (see Command
           modifiers), and manages the error number !.  The first argument
           must be either [+[+[+]]]e[ncode], d[ecode], s[kin] or skinl[ist]
           and specifies the operation to perform on the rest of the line.
           Decoding will show how a standard-compliant MUA will display the
           given argument, which should be an email address.  Please be aware
           that most MUAs have difficulties with the address standards, and
           vary wildly when (comments) in parenthesis, ``double-quoted''
           strings, or quoted-pairs, as below, become involved.  [v15 behav-
           iour may differ] S-nail currently does not perform decoding when
           displaying addresses.
           Skinning is identical to decoding but only outputs the plain
           address, without any string, comment etc. components.  Another dif-
           ference is that it may fail with the error number ! set to
           ^ERR-INVAL if decoding fails to find a(n) (valid) email address, in
           which case the unmodified input will be output again.
           skinlist first performs a skin operation, and thereafter checks a
           valid address for whether it is a registered mailing list (see
           mlist and mlsubscribe), eventually reporting that state in the
           error number ! as ^ERR-EXIST.  (This state could later become over-
           written by an I/O error, though.)
           Encoding supports four different modes, lesser automated versions
           can be chosen by prefixing one, two or three plus signs: the stan-
           dard imposes a special meaning on some characters, which thus have
           to be transformed to so-called quoted-pairs by pairing them with a
           reverse solidus `\' in order to remove the special meaning; this
           might change interpretation of the entire argument from what has
           been desired, however!  Specify one plus sign to remark that paren-
           thesis shall be left alone, two for not turning double quotation
           marks into quoted-pairs, and three for also leaving any user-speci-
           fied reverse solidus alone.  The result will always be valid, if a
           successful exit status is reported ([v15 behaviour may differ] the
           current parser fails this assertion for some constructs).  [v15 be-
           haviour may differ] Addresses need to be specified in between angle
           brackets `<', `>' if the construct becomes more difficult, other-
           wise the current parser will fail; it is not smart enough to guess
           right.
                 ? addrc enc "Hey, you",<diet AT exam.ple>\ out\ there
                 "\"Hey, you\", \\ out\\ there" <diet AT exam.ple>
                 ? addrc d "\"Hey, you\", \\ out\\ there" <diet AT exam.ple>
                 "Hey, you", \ out\ there <diet AT exam.ple>
                 ? addrc s "\"Hey, you\", \\ out\\ there" <diet AT exam.ple>
                 diet AT exam.ple
     alias, unalias
           [Only new quoting rules](a, una) Define or list, and remove,
           respectively, address aliases, which are a method of creating per-
           sonal distribution lists that map a single name to none to multiple
           receivers, to be expanded after Compose mode is left; the expansion
           correlates with metoo.  The latter command removes all given
           aliases, the special name asterisk `*' will remove all existing
           aliases.  When used without arguments the former shows a list of
           all currently known aliases, with one argument only the target(s)
           of the given one.  When given two arguments, hyphen-minus `-' being
           the first, the target(s) of the second is/are expanded recursively.
           In all other cases the given alias is newly defined, or will be
           appended to: arguments must either be themselves valid alias names,
           or any other address type (see On sending mail, and non-interactive
           mode).  Recursive expansion of aliases can be prevented by prefix-
           ing the desired argument with the modifier reverse solidus \.  A
           valid alias name conforms to mta-aliases syntax, but follow-up
           characters can also be the number sign `#', colon `:', commercial
           at `@,' exclamation mark `!', period `.' as well as ``any character
           that has the high bit set''.  The dollar sign `$' may be the last
           character.  The number sign `#' may need Shell-style argument
           quoting.
           [v15 behaviour may differ] Unfortunately the colon is currently not
           supported, as it interferes with normal address parsing rules.
           [v15 behaviour may differ] Such high bit characters will likely
           cause warnings at the moment for the same reasons why colon is
           unsupported; also, in the future locale dependent character set
           validity checks will be performed.
                 ? alias cohorts  bill jkf mark kridle@ucbcory ~/cohorts.mbox
                 ? alias mark  mark AT exam.ple
                 ? set mta-aliases=/etc/aliases
     alternates, unalternates
           [Only new quoting rules] (alt) Manage a list of alternate addresses
           or names of the active user, members of which will be removed from
           recipient lists (except one).  There is a set of implicit alter-
           nates which is formed of the values of LOGNAME, from, sender and
           reply-to.  from will not be used if sender is set.  The latter com-
           mand removes the given list of alternates, the special name `*'
           will discard all existing alternate names.
           The former command manages the error number !.  It shows the cur-
           rent set of alternates when used without arguments; in this mode
           only it also supports vput (see Command modifiers).  Otherwise the
           given arguments (after being checked for validity) are appended to
           the list of alternate names; in posix mode they replace that list
           instead.
     answered, unanswered
           Take a message lists and mark each message as (not) having been
           answered.  Messages will be marked answered when being replyd to
           automatically if the markanswered variable is set.  See the section
           Message states.
     bind, unbind
           [Option][Only new quoting rules] The bind command extends the MLE
           (see On terminal control and line editor) with freely configurable
           key bindings.  The latter command removes from the given context
           the given key binding, both of which may be specified as a wildcard
           `*', so that `unbind * *' will remove all bindings of all contexts.
           Due to initialization order unbinding will not work for built-in
           key bindings upon program startup, however: please use
           line-editor-no-defaults for this purpose instead.
           With zero arguments, or with a context name the former command
           shows all key bindings (of the given context; an asterisk `*' will
           iterate over all contexts); a more verbose listing will be produced
           if either of debug or verbose are set.  With two or more arguments
           a specific binding is shown, or (re)established: the first argument
           is the context to which the binding shall apply, the second argu-
           ment is a comma-separated list of the ``keys'' which form the bind-
           ing.  Further arguments will be joined to form the expansion, and
           cause the binding to be created or updated.  To indicate that a
           binding shall not be auto-committed, but that the expansion shall
           instead be furtherly editable by the user, a commercial at `@'
           (that will be removed) can be placed last in the expansion, from
           which leading and trailing whitespace will finally be removed.
           Reverse solidus cannot be used as the last character of expansion.
           An empty expansion will be rejected.
           Contexts define when a binding applies, i.e., a binding will not be
           seen unless the context for which it is defined for is currently
           active.  This is not true for the shared binding `base', which is
           the foundation for all other bindings and as such always applies,
           its bindings, however, only apply secondarily.  The available con-
           texts are the shared `base', the `default' context which is used in
           all not otherwise documented situations, and `compose', which
           applies only to Compose mode.
           Bindings are specified as a comma-separated list of byte-sequences,
           where each list entry corresponds to one ``key'' (press).  Byte
           sequence boundaries will be forcefully terminated after
           bind-inter-byte-timeout milliseconds, whereas key sequences can be
           timed out via bind-inter-key-timeout.  A list entry may, indicated
           by a leading colon character `:', also refer to the name of a ter-
           minal capability; several dozen names are compiled in and may be
           specified either by their terminfo(5), or, if existing, by their
           termcap(5) name, regardless of the actually used [Option]al termi-
           nal control library.  But any capability may be used, as long as
           the name is resolvable by the [Option]al control library, or was
           defined via the internal variable termcap.  Input sequences are not
           case-normalized, an exact match is required to update or remove a
           binding.  It is advisable to use an initial escape or other control
           character (like `\cA') for user (as opposed to purely terminal
           capability based) bindings in order to avoid ambiguities; it also
           reduces search time.  Examples:
                 ? bind base a,b echo one
                 ? bind base $'\E',d mle-snarf-word-fwd # Esc(ape)
                 ? bind base $'\E',$'\c?' mle-snarf-word-bwd # Esc,Delete
                 ? bind default $'\cA',:khome,w 'echo Editable binding@'
                 ? bind default a,b,c rm -irf / @  # Also editable
                 ? bind default :kf1 File %
                 ? bind compose :kf1 ~v
           Note that the entire comma-separated list is first parsed (over) as
           a shell-token with whitespace as the field separator, then parsed
           and expanded for real with comma as the field separator, therefore
           whitespace needs to be properly quoted, see Shell-style argument
           quoting.  Using Unicode reverse solidus escape sequences renders a
           binding defunctional if the locale does not support Unicode (see
           Character sets), and using terminal capabilities does so if no
           (corresponding) terminal control support is (currently) available.
           Adding, deleting or modifying a key binding invalidates the inter-
           nal prebuilt lookup tree, it will be recreated as necessary: this
           process will be visualized in most verbose as well as in debug
           mode.
           The following terminal capability names are built-in and can be
           used in terminfo(5) or (if available) the two-letter termcap(5)
           notation.  See the respective manual for a list of capabilities.
           The program infocmp(1) can be used to show all the capabilities of
           TERM or the given terminal type; using the -x flag will also show
           supported (non-standard) extensions.
           kbs or kb       Backspace.
           kdch1 or kD     Delete character.
           kDC or *4       -- shifted variant.
           kel or kE       Clear to end of line.
           kext or @9      Exit.
           kich1 or kI     Insert character.
           kIC or #3       -- shifted variant.
           khome or kh     Home.
           kHOM or #2      -- shifted variant.
           kend or @7      End.
           knp or kN       Next page.
           kpp or kP       Previous page.
           kcub1 or kl     Left cursor (with more modifiers: see below).
           kLFT or #4      -- shifted variant.
           kcuf1 or kr     Right cursor (ditto).
           kRIT or %i      -- shifted variant.
           kcud1 or kd     Down cursor (ditto).
           kDN             -- shifted variant (only terminfo).
           kcuu1 or ku     Up cursor (ditto).
           kUP             -- shifted variant (only terminfo).
           kf0 or k0       Function key 0.  Add one for each function key up
                           to kf9 and k9, respectively.
           kf10 or k;      Function key 10.
           kf11 or F1      Function key 11.  Add one for each function key up
                           to kf19 and F9, respectively.
           Some terminals support key-modifier combination extensions, e.g.,
           `Alt+Shift+xy'.  For example, the delete key, kdch1: in its shifted
           variant, the name is mutated to kDC, then a number is appended for
           the states `Alt' (kDC3), `Shift+Alt' (kDC4), `Control' (kDC5),
           `Shift+Control' (kDC6), `Alt+Control' (kDC7), finally
           `Shift+Alt+Control' (kDC8).  The same for the left cursor key,
           kcub1: KLFT, KLFT3, KLFT4, KLFT5, KLFT6, KLFT7, KLFT8.
     call  [Only new quoting rules] Calls the given macro, which must have
           been created via define (see there for more), otherwise an
           ^ERR-NOENT error occurs.  Calling macros recursively will at some
           time excess the stack size limit, causing a hard program abortion;
           if recursively calling a macro is the last command of the current
           macro, consider to use the command xcall, which will first release
           all resources of the current macro before replacing the current
           macro with the called one.
     call_if
           Identical to call if the given macro has been created via define,
           but does not fail nor warn if the macro does not exist.
     cd    Synonym for chdir.
     certsave
           [Option] Only applicable to S/MIME signed messages.  Takes an
           optional message list and a filename and saves the certificates
           contained within the message signatures to the named file in both
           human-readable and PEM format.  The certificates can later be used
           to send encrypted messages to the respective message senders by
           setting smime-encrypt-USER@HOST variables.
     charsetalias, uncharsetalias
           [Only new quoting rules] Manage alias mappings for (conversion of)
           Character sets.  Alias processing is not performed for INTERNAL
           VARIABLES, for example charset-8bit, and mappings are ineffective
           if character set conversion is not available (features does not
           announce `,+iconv,').  Expansion happens recursively for cases
           where aliases point to other aliases (built-in loop limit: 8).
           The latter command deletes all aliases given as arguments, or all
           at once when given the asterisk `*'.  The former shows the list of
           all currently defined aliases if used without arguments, or the
           target of the given single argument; when given two arguments,
           hyphen-minus `-' being the first, the second is instead expanded
           recursively.  In all other cases the given arguments are treated as
           pairs of character sets and their desired target alias name, creat-
           ing new or updating already existing aliases.
     chdir
           [Only new quoting rules](ch) Change the working directory to HOME
           or the given argument.  Synonym for cd.
     collapse, uncollapse
           Only applicable to `thread'ed sort mode.  Takes a message list and
           makes all replies to these messages invisible in header summaries,
           except for `new' messages and the ``dot''.  Also when a message
           with collapsed replies is displayed, all of these are automatically
           uncollapsed.  The latter command undoes collapsing.
     colour, uncolour
           [Option][Only new quoting rules] Manage colour mappings of and for
           a Coloured display.  Without arguments the former shows all cur-
           rently defined mappings.  Otherwise a colour type is expected
           (case-insensitively), it must be one of `256' for 256-colour termi-
           nals, `8', `ansi' or `iso' for the standard 8-colour ANSI / ISO
           6429 colour palette, and `1' or `mono' for monochrome terminals,
           which only support (some) font attributes.  Without further argu-
           ments the list of all currently defined mappings of the given type
           is shown (here the special `all' or `*' also show all currently
           defined mappings).
           Otherwise the second argument defines the mappable slot, the third
           argument a (comma-separated list of) colour and font attribute
           specification(s), and the optionally supported fourth argument can
           be used to specify a precondition: if conditioned mappings exist
           they are tested in (creation) order unless a (case-insensitive)
           match has been found, and the default mapping (if any has been
           established) will only be chosen as a last resort.  The types of
           available preconditions depend on the mappable slot, the following
           of which exist:
           Mappings prefixed with `mle-' are used for the [Option]al built-in
           Mailx-Line-Editor (MLE, see On terminal control and line editor)
           and do not support preconditions.
           mle-position   This mapping is used for the position indicator that
                          is visible when a line cannot be fully displayed on
                          the screen.
           mle-prompt     Used for the prompt.
           mle-error      Used for the occasionally appearing error indicator
                          that is joined onto prompt.  [v15 behaviour may dif-
                          fer] Also used for error messages written on stan-
                          dard error .
           Mappings prefixed with `sum-' are used in header summaries, and
           they all understand the preconditions `dot' (the current message)
           and `older' for elder messages (only honoured in conjunction with
           datefield-markout-older).
           sum-dotmark    This mapping is used for the ``dotmark'' that can be
                          created with the `%>' or `%<' formats of the vari-
                          able headline.
           sum-header     For the complete header summary line except the
                          ``dotmark'' and the thread structure.
           sum-thread     For the thread structure which can be created with
                          the `%i' format of the variable headline.
           Mappings prefixed with `view-' are used when displaying messages.
           view-from_     This mapping is used for so-called `From_' lines,
                          which are MBOX file format specific header lines
                          (also see mbox-rfc4155).
           view-header    For header lines.  A comma-separated list of headers
                          to which the mapping applies may be given as a pre-
                          condition; if the [Option]al regular expression sup-
                          port is available then if any of the magic regular
                          expression characters is seen the precondition will
                          be evaluated as (an extended) one.
           view-msginfo   For the introductional message info line.
           view-partinfo  For MIME part info lines.
           The following (case-insensitive) colour definitions and font
           attributes are understood, multiple of which can be specified in a
           comma-separated list:
           ft=  a font attribute: `bold', `reverse' or `underline'.  It is
                possible (and often applicable) to specify multiple font
                attributes for a single mapping.
           fg=  foreground colour attribute, in order (numbers 0 - 7) `black',
                `red', `green', `brown', `blue', `magenta', `cyan' or `white'.
                To specify a 256-colour mode a decimal number colour specifi-
                cation in the range 0 to 255, inclusive, is supported, and
                interpreted as follows:
                0 - 7      the standard ISO 6429 colours, as above.
                8 - 15     high intensity variants of the standard colours.
                16 - 231   216 colours in tuples of 6.
                232 - 255  grayscale from black to white in 24 steps.
                      #!/bin/sh -
                      fg() { printf "\033[38;5;${1}m($1)"; }
                      bg() { printf "\033[48;5;${1}m($1)"; }
                      i=0
                      while [ $i -lt 256 ]; do fg $i; i=$(($i + 1)); done
                      printf "\033[0m\n"
                      i=0
                      while [ $i -lt 256 ]; do bg $i; i=$(($i + 1)); done
                      printf "\033[0m\n"
           bg=  background colour attribute (see fg= for possible values).
           The command uncolour will remove for the given colour type (the
           special type `*' selects all) the given mapping; if the optional
           precondition argument is given only the exact tuple of mapping and
           precondition is removed.  The special name `*' will remove all map-
           pings (no precondition allowed), thus `uncolour * *' will remove
           all established mappings.
     commandalias, uncommandalias
           [Only new quoting rules] Define or list, and remove, respectively,
           command aliases.  An (command)alias can be used everywhere a normal
           command can be used, but always takes precedence: any arguments
           that are given to the command alias are joined onto the alias
           expansion, and the resulting string forms the command line that is,
           in effect, executed.  The latter command removes all given aliases,
           the special name asterisk `*' will remove all existing aliases.
           When used without arguments the former shows a list of all cur-
           rently known aliases, with one argument only the expansion of the
           given one.
           With two or more arguments a command alias is defined or updated:
           the first argument is the name under which the remaining command
           line should be accessible, the content of which can be just about
           anything.  An alias may itself expand to another alias, but to
           avoid expansion loops further expansion will be prevented if an
           alias refers to itself or if an expansion depth limit is reached.
           Explicit expansion prevention is available via reverse solidus \,
           one of the Command modifiers.
                 ? commandalias xx
                 s-nail: `commandalias': no such alias: xx
                 ? commandalias xx echo hello,
                 ? commandalias xx
                 commandalias xx 'echo hello,'
                 ? xx
                 hello,
                 ? xx world
                 hello, world
     Copy  (C) Similar to copy, but copy the messages to a file named after
           the local part of the sender of the first message instead of taking
           a filename argument; outfolder is inspected to decide on the actual
           storage location.
     copy  (c) Copy messages to the named file and do not mark them as being
           saved; otherwise identical to save.
     csop  [Only new quoting rules] A multiplexer command which provides C-
           style string operations on 8-bit bytes without a notion of locale
           settings and character sets, effectively assuming ASCII data.  For
           numeric and other operations refer to vexpr.  vput, one of the
           Command modifiers, is supported.  The error result is `-1' for
           usage errors and numeric results, the empty string otherwise; miss-
           ing data errors, as for unsuccessful searches, result in the !
           error number being set to ^ERR-NODATA.  Where the question mark `?'
           modifier suffix is supported, a case-insensitive (ASCII mapping)
           operation mode is supported; the keyword `case' is optional so that
           `find?' and `find?case' are identical.
           length    Queries the length of the given argument.
           hash, hash32 Calculates a hash value of the given argument.  The
                     latter will return a 32-bit result regardless of host
                     environment.  `?' modifier suffix is supported.  These
                     use Chris Torek's hash algorithm, the resulting hash
                     value is bit mixed as shown by Bret Mulvey.
           find      Search for the second in the first argument.  Shows the
                     resulting 0-based offset shall it have been found.  `?'
                     modifier suffix is supported.
           substring Creates a substring of its first argument.  The optional
                     second argument is the 0-based starting offset, a nega-
                     tive one counts from the end; the optional third argument
                     specifies the length of the desired result, a negative
                     length leaves off the given number of bytes at the end of
                     the original string; by default the entire string is
                     used.  This operation tries to work around faulty argu-
                     ments (set verbose for error logs), but reports them via
                     the error number ! as ^ERR-OVERFLOW.
           trim      Trim away whitespace characters from both ends of the
                     argument.
           trim-front Trim away whitespace characters from the begin of the
                     argument.
           trim-end  Trim away whitespace characters from the end of the argu-
                     ment.
     cwd   Show the name of the current working directory, as reported by
           getcwd(3).  Supports vput (see Command modifiers).  The return sta-
           tus is tracked via ?.
     Decrypt
           [Option] For unencrypted messages this command is identical to
           Copy; Encrypted messages are first decrypted, if possible, and then
           copied.
     decrypt
           [Option] For unencrypted messages this command is identical to
           copy; Encrypted messages are first decrypted, if possible, and then
           copied.
     define, undefine
           The latter command deletes the given macro, the special name `*'
           will discard all existing macros.  Deletion of (a) macro(s) can be
           performed from within running (a) macro(s), including self-dele-
           tion.  Without arguments the former command prints the current list
           of macros, including their content, otherwise it defines a macro,
           replacing an existing one of the same name as applicable.
           A defined macro can be invoked explicitly by using the call,
           call_if and xcall commands, or implicitly if a macro hook is trig-
           gered, for example a folder-hook.  Execution of a macro body can be
           stopped from within by calling return.
           Temporary macro block-scope variables can be created or deleted
           with the local command modifier in conjunction with the commands
           set and unset, respectively.  To enforce unrolling of changes made
           to (global) INTERNAL VARIABLES the command localopts can be used
           instead; its covered scope depends on how (i.e., ``as what'': nor-
           mal macro, folder hook, hook, account switch) the macro is invoked.
           Inside a called macro, the given positional parameters are implic-
           itly local to the macro's scope, and may be accessed via the vari-
           ables *, @, # and 1 and any other positive unsigned decimal number
           less than or equal to #.  Positional parameters can be shifted, or
           become completely replaced, removed etc. via vpospar.  A helpful
           command for numeric computation and string evaluations is vexpr,
           csop offers C-style byte string operations.
                 define name {
                   command1
                   command2
                   ...
                   commandN
                 }
                 define exmac {
                   echo Parameter 1 of ${#} is ${1}, all: ${*} / ${@}
                   return 1000 0
                 }
                 call exmac Hello macro exmac!
                 echo ${?}/${!}/${^ERRNAME}
     delete, undelete
           (d, u) Marks the given message list as being or not being
           `deleted', respectively; if no argument has been specified then the
           usual search for a visible message is performed, as documented for
           Message list arguments, showing only the next input prompt if the
           search fails.  Deleted messages will neither be saved in the
           secondary mailbox MBOX nor will they be available for most other
           commands.  If the autoprint variable is set, the new ``dot'' or the
           last message restored, respectively, is automatically typed; also
           see dp, dt.
     digmsg
           [Only new quoting rules] Digging (information out of) messages is
           possible through digmsg objects, which can be created for the given
           message number; in Compose mode the hyphen-minus `-' will instead
           open the message that is being composed.  If a hyphen-minus is
           given as the optional third argument then output will be generated
           on the standard output channel instead of being subject to consump-
           tion by the readall (or read and readsh) command(s).  Note: output
           must be consumed before normal processing can continue; for digmsg
           objects this means each command output has to be read until the end
           of file (EOF) state occurs.
           The objects may be removed again by giving the same identifier used
           for creation; this step could be omitted: objects will be automati-
           cally closed when the active folder (mailbox) or the compose mode
           is left, respectively.  In all other use cases the second argument
           is an object identifier, and the third and all following arguments
           are interpreted as via ~^ (see COMMAND ESCAPES):
                 ? vput = msgno; digmsg create $msgno
                 ? digmsg $msgno header list;   readall x;   echon $x
                 210 Subject From To Message-ID References In-Reply-To
                 ? digmsg $msgno header show Subject;readall x;echon $x
                 212 Subject
                 'Hello, world'
                 ? digmsg remove $msgno
     discard
           (di) Identical to ignore.  Superseded by the multiplexer
           headerpick.
     dp, dt
           Delete the given messages and automatically type the new ``dot'' if
           one exists, regardless of the setting of autoprint.
     dotmove
           Move the ``dot'' up or down by one message when given `+' or `-'
           argument, respectively.
     draft, undraft
           Take message lists and mark each given message as being draft, or
           not being draft, respectively, as documented in the section Message
           states.
     echo  [Only new quoting rules](ec) Print the given strings, equivalent to
           the shell utility echo(1), that is, Shell-style argument quoting
           expansion is performed and, different to the otherwise identical
           echon, a trailing newline is echoed.  vput as documented in Command
           modifiers is supported, and the error number ! is managed: if data
           is stored in a variable then the return value reflects the length
           of the result string in case of success and is `-1' on error.
           Remarks: this command traditionally (in BSD Mail) also performed
           Filename transformations, which is standard incompatible and hard
           to handle because quoting transformation patterns is not possible;
           the subcommand file-expand of vexpr can be used to expand file-
           names.
     echoerr
           [Only new quoting rules] Identical to echo, but the message is
           written to standard error, and prefixed by log-prefix.  Also see
           echoerrn.  In interactive sessions the [Option]al message ring
           queue for errors will be used instead, if available and vput was
           not used.
     echon
           [Only new quoting rules] Identical to echo, but does not write or
           store a trailing newline.
     echoerrn
           [Only new quoting rules] Identical to echoerr, but does not write
           or store a trailing newline.
     edit  (e) Point the text EDITOR at each message from the given list in
           turn.  Modified contents are discarded unless the writebackedited
           variable is set, and are not used unless the mailbox can be written
           to and the editor returns a successful exit status.  visual can be
           used instead for a more display oriented editor.
     elif  Part of the if (see there for more), elif, else, endif conditional
           -- if the condition of a preceding if was false, check the follow-
           ing condition and execute the following block if it evaluates true.
     else  (el) Part of the if (see there for more), elif, else, endif condi-
           tional -- if none of the conditions of the preceding if and elif
           commands was true, the else block is executed.
     endif
           (en) Marks the end of an if (see there for more), elif, else, endif
           conditional execution block.
     environ
           [Only new quoting rules] There is a strict separation in between
           INTERNAL VARIABLES and the program ENVIRONMENT, which is inherited
           by child processes.  Some variables of the latter are however vivid
           for program operation, their purpose is known, therefore they have
           been integrated transparently into handling of the former, as
           accessible via set and unset.  To integrate any other environment
           variable, and/or to export internal variables into the process
           environment where they normally are not, a link needs to become
           established with this command, for example
                 environ link PERL5LIB TZ
           Afterwards changing such variables with set will cause automatic
           updates of the environment, too.  Sufficient system support pro-
           vided (it was in BSD as early as 1987, and is standardized since
           Y2K) removing such variables with unset will remove them also from
           the environment, but in any way the knowledge they ever have been
           linked will be lost.  This implies that localopts may cause loss of
           such links.
           The subcommand unlink removes an existing link without otherwise
           touching variables, the set and unset subcommands are identical to
           set and unset, but additionally update the program environment
           accordingly; removing a variable breaks any freely established
           link.
     errors
           [Option] As console user interfaces at times scroll error messages
           by too fast and/or out of scope, data can additionally be sent to
           an error queue manageable by this command: show or no argument will
           display and clear the queue, clear will only clear it.  As the
           queue becomes filled with errors-limit entries the eldest entries
           are being dropped.  There are also the variables ^ERRQUEUE-COUNT
           and ^ERRQUEUE-EXISTS.
     eval  [Only new quoting rules] Construct a command by concatenating the
           arguments, separated with a single space character, and then evalu-
           ate the result.  This command passes through the exit status ? and
           error number ! of the evaluated command; also see call.
                 define xxx {
                   echo "xxx arg <$1>"
                   shift
                   if $# -gt 0
                     \xcall xxx "$@"
                   endif
                 }
                 define yyy {
                   eval "$@ ' ball"
                 }
                 call yyy '\call xxx' "b\$'\t'u ' "
                 call xxx arg <b      u>
                 call xxx arg <  >
                 call xxx arg <ball>
     exit  (ex or x) Exit from S-nail without changing the active mailbox and
           skip any saving of messages in the secondary mailbox MBOX, as well
           as a possibly tracked line editor history-file.  A possibly set
           on-account-cleanup will be invoked, however.  The optional status
           number argument will be passed through to exit(3).  [v15 behaviour
           may differ] For now it can happen that the given status will be
           overwritten, later this will only occur if a later error needs to
           be reported onto an otherwise success indicating status.
     File  (Fi) Like folder, but open the mailbox read-only.
     file  (fi) See folder.
     filetype, unfiletype
           [Only new quoting rules] Define, list, and remove, respectively,
           file handler hooks, which provide (shell) commands that enable
           S-nail to load and save MBOX files from and to files with the reg-
           istered file extensions, as shown and described for folder.  The
           extensions are used case-insensitively, yet the auto-completion
           feature of for example folder will only work case-sensitively.  An
           intermediate temporary file will be used to store the expanded
           data.  The latter command will remove hooks for all given exten-
           sions, asterisk `*' will remove all existing handlers.
           When used without arguments the former shows a list of all cur-
           rently defined file hooks, with one argument the expansion of the
           given alias.  Otherwise three arguments are expected, the first
           specifying the file extension for which the hook is meant, and the
           second and third defining the load- and save commands to deal with
           the file type, respectively, both of which must read from standard
           input and write to standard output.  Changing hooks will not affect
           already opened mailboxes ([v15 behaviour may differ] except below).
           [v15 behaviour may differ] For now too much work is done, and files
           are oftened read in twice where once would be sufficient: this can
           cause problems if a filetype is changed while such a file is
           opened; this was already so with the built-in support of .gz etc.
           in Heirloom, and will vanish in v15.  [v15 behaviour may differ]
           For now all handler strings are passed to the SHELL for evaluation
           purposes; in the future a `!' prefix to load and save commands may
           mean to bypass this shell instance: placing a leading space will
           avoid any possible misinterpretations.
                 ? filetype bz2 'bzip2 -dc' 'bzip2 -zc' \
                     gz 'gzip -dc' 'gzip -c'  xz 'xz -dc' 'xz -zc' \
                     zst 'zstd -dc' 'zstd -19 -zc' \
                     zst.pgp 'gpg -d | zstd -dc' 'zstd -19 -zc | gpg -e'
                 ? set record=+sent.zst.pgp
     flag, unflag
           Take message lists and mark the messages as being flagged, or not
           being flagged, respectively, for urgent/special attention.  See the
           section Message states.
     Folder
           (Fold) Like folder, but open the mailbox read-only.
     folder
           (fold) Open a new, or show status information of the current mail-
           box.  If an argument is given, changes (such as deletions) will be
           written out, a new mailbox will be opened, the internal variables
           mailbox-resolved and mailbox-display will be updated, a set accord-
           ing folder-hook is executed, and optionally a summary of headers is
           displayed if the variable header is set.
           Filename transformations will be applied to the name argument, and
           `protocol://' prefixes are, i.e., URL (see On URL syntax and
           credential lookup) syntax is understood, as in
           `mbox:///tmp/somefolder'.  If a protocol prefix is used the mailbox
           type is fixated, otherwise opening none-existing folders uses the
           protocol defined in newfolders.
           For the protocols mbox and file (MBOX database), as well as eml
           (electronic mail message [v15 behaviour may differ] read-only) the
           list of all registered filetypes is traversed to check whether
           hooks shall be used to load (and save) data from (and to) the given
           name.  Changing hooks will not affect already opened mailboxes.
           For example, the following creates hooks for the gzip(1) compres-
           sion tool and a combined compressed and encrypted format:
                 ? filetype \
                     gzip 'gzip -dc' 'gzip -c' \
                     zst.pgp 'gpg -d | zstd -dc' 'zstd -19 -zc | gpg -e'
           For historic reasons filetypes provide limited (case-sensitive)
           auto-completion capabilities.  For example `mbox.gz' will be found
           for `? file mbox', provided that corresponding handlers are
           installed.  It will neither find `mbox.GZ' nor `mbox.Gz' however,
           but an explicit `? file mbox.GZ' will find and use the handler for
           `gz'.  [v15 behaviour may differ] The latter mode can only be used
           for MBOX files.
           EML files consist of only one mail message, [v15 behaviour may dif-
           fer] and can only be opened read-only.  When reading MBOX files
           tolerant POSIX rules are used by default.  Invalid message bound-
           aries that can be found quite often in historic MBOX files will be
           complained about (even more with debug): in this case the method
           described for mbox-rfc4155 can be used to create a valid MBOX data-
           base from the invalid input.
           MBOX databases and EML files will always be protected via file-
           region locks (fcntl(2)) during file operations to protect against
           concurrent modifications.  [Option] An MBOX inbox (MAIL) or primary
           system mailbox will also be protected by so-called dotlock files,
           the traditional way of mail spool file locking: for any file `x' a
           lock file `x.lock' will be created during the synchronization, in
           the same directory and with the same user and group identities as
           the file of interest -- as necessary created by an external privi-
           leged dotlock helper.  dotlock-disable disables dotlock files.
           Also see FAQ: Howto handle stale dotlock files.
           [Option] If no protocol has been fixated, and name refers to a
           directory with the subdirectories `tmp', `new' and `cur', then it
           is treated as a folder in ``Maildir'' format.  The maildir format
           stores each message in its own file, and has been designed so that
           file locking is not necessary when reading or writing files.
           [Option]ally URLs can be used to access network resources, securely
           via Encrypted network communication, if so supported.  Network com-
           munication socket timeouts are configurable via
           socket-connect-timeout.  All network traffic may be proxied over a
           SOCKS server via socks-proxy.
                 [v15-compat] protocol://[user[:password]@]host[:port][/path]
                 [no v15-compat] protocol://[user@]host[:port][/path]
           [Option]ally supported network protocols are pop3 (POP3) and pop3s
           (POP3 with TLS encrypted transport), imap and imaps.  The [/path]
           part is valid only for IMAP; there it defaults to INBOX.  Network
           URLs require a special encoding as documented in the section On URL
           syntax and credential lookup.
     folders
           Lists the names of all folders below the given argument or folder.
           For file-based protocols LISTER will be used for display purposes.
     Followup, followup
           (Compose mode)(F,fo) Similar to Reply, and reply, respectively, but
           save the message in a file named after the local part of the
           (first) recipient's address, possibly overwriting record, and hon-
           ouring outfolder.  Also see Copy and Save.
     Forward
           (Compose mode) Similar to forward, but saves the message in a file
           named after the local part of the recipient's address (instead of
           in record).
     forward
           (Compose mode) Take a message list and the address of a recipient,
           subject to fullnames, to whom the messages are sent.  The text of
           the original message is included in the new one, enclosed by the
           values of forward-inject-head and forward-inject-tail.
           content-description-forwarded-message is inspected.  The list of
           included headers can be filtered with the `forward' slot of the
           white- and blacklisting command headerpick.  Only the first part of
           a multipart message is included but for forward-as-attachment.
           This may generate the errors ^ERR-DESTADDRREQ if no receiver has
           been specified, or was rejected by expandaddr policy, ^ERR-IO if an
           I/O error occurs, ^ERR-NOTSUP if a necessary character set conver-
           sion fails, and ^ERR-INVAL for other errors.  It can also fail with
           errors of Specifying messages.  Any error stops processing of fur-
           ther messages.
     from  (f) Takes a list of message specifications and displays a summary
           of their message headers, exactly as via headers, making the first
           message of the result the new ``dot'' (the last message if showlast
           is set).  An alias of this command is search.  Also see Specifying
           messages.
     Fwd   [Obsolete] Alias for Forward.
     fwd   [Obsolete] Alias for forward.
     fwdignore
           [Obsolete] Superseded by the multiplexer headerpick.
     fwdretain
           [Obsolete] Superseded by the multiplexer headerpick.
     ghost, unghost
           [Obsolete] Replaced by commandalias, uncommandalias.
     headerpick, unheaderpick
           [Only new quoting rules] Multiplexer command to manage white- and
           blacklisting selections of header fields for a variety of applica-
           tions.  Without arguments the set of contexts that have settings is
           displayed.  When given arguments, the first argument is the context
           to which the command applies, one of (case-insensitive) `type' for
           display purposes (for example type), `save' for selecting which
           headers shall be stored persistently when save, copy, move or even
           decrypting messages (note that MIME related etc. header fields
           should not be ignored in order to not destroy usability of the mes-
           sage in this case), `forward' for stripping down messages when
           forwarding message (has no effect if forward-as-attachment is set),
           and `top' for defining user-defined set of fields for the command
           top.
           The current settings of the given context are displayed if it is
           the only argument.  A second argument denotes the type of restric-
           tion that is to be chosen, it may be (a case-insensitive prefix of)
           `retain' or `ignore' for white- and blacklisting purposes, respec-
           tively.  Establishing a whitelist suppresses inspection of the cor-
           responding blacklist.
           If no further argument is given the current settings of the given
           type will be displayed, otherwise the remaining arguments specify
           header fields, which [Option]ally may be given as regular expres-
           sions, to be added to the given type.  The special wildcard field
           (asterisk, `*') will establish a (fast) shorthand setting which
           covers all fields.
           The latter command always takes three or more arguments and can be
           used to remove selections, i.e., from the given context, the given
           type of list, all the given headers will be removed, the special
           argument `*' will remove all headers.
     headers
           (h) Show the current group of headers, the size of which depends on
           the variable screen in interactive mode, and the format of which
           can be defined with headline.  If a message-specification is given
           the group of headers containing the first message therein is shown
           and the message at the top of the screen becomes the new ``dot'';
           the last message is targeted if showlast is set.
     help  (hel) A synonym for ?.
     history
           [Option] Without arguments or when given show all history entries
           are shown (this mode also supports a more verbose output).  load
           will replace the list of entries with the content of history-file,
           and save will dump all entries to said file, replacing former con-
           tent, and clear will delete all entries.  The argument can also be
           a signed decimal NUMBER, which will select and evaluate the respec-
           tive history entry, and move it to the top of the history; a nega-
           tive number is used as an offset to the current command so that
           `-1' will select the last command, the history top, whereas delete
           will delete all given entries (:NUMBER:).  Also see On terminal
           control and line editor.
     hold  (ho, also preserve) Takes a message list and marks each message
           therein to be saved in the user's system inbox instead of in the
           secondary mailbox MBOX.  Does not override the delete command.
           S-nail deviates from the POSIX standard with this command, because
           a next command issued after hold will display the following mes-
           sage, not the current one.
     if    (i) Part of the if, elif, else, endif conditional execution con-
           struct -- if the given condition is true then the encapsulated
           block is executed.  The POSIX standard only supports the (case-
           insensitive) conditions `r'eceive and `s'end, the remaining are
           non-portable extensions.  [v15 behaviour may differ] In conjunction
           with the wysh command prefix(es) Shell-style argument quoting and
           more test operators are available.
                 if receive
                   commands ...
                 else
                   commands ...
                 endif
           Further (case-insensitive) one-argument conditions are `t'erminal
           which evaluates to true in interactive terminal sessions (running
           with standard input or standard output attached to a terminal, and
           none of the ``quickrun'' command line options -e, -H and -L have
           been used), as well as any boolean value (see INTERNAL VARIABLES
           for textual boolean representations) to mark an enwrapped block as
           ``never execute'' or ``always execute''.  (Remarks: condition syn-
           tax errors skip all branches until endif.)
           [no v15-compat] and without wysh: It is possible to check INTERNAL
           VARIABLES as well as ENVIRONMENT variables for existence or compare
           their expansion against a user given value or another variable by
           using the `$' (``variable next'') conditional trigger character; a
           variable on the right hand side may be signalled using the same
           mechanism.  Variable names may be enclosed in a pair of matching
           braces.  When this mode has been triggered, several operators are
           available ([v15-compat] and wysh: they are always available, and
           there is no trigger: variables will have been expanded by the
           shell-compatible parser before the if etc. command sees them).
           [v15-compat] Two argument conditions.  Variables can be tested for
           existence and expansion: `-N' will test whether the given variable
           exists, so that `-N editalong' will evaluate to true when editalong
           is set, whereas `-Z editalong' will if it is not.  `-n
           "$editalong"' will be true if the variable is set and expands to a
           non-empty string, `-z $'\$editalong'' only if the expansion is
           empty, whether the variable exists or not.  The remaining condi-
           tions take three arguments.
           Integer operators treat the arguments on the left and right hand
           side of the operator as integral numbers and compare them arith-
           metically.  It is an error if any of the operands is not a valid
           integer, an empty argument (which implies it had been quoted) is
           treated as if it were 0.  Via the question mark `?' modifier suffix
           a saturated operation mode is available where numbers will linger
           at the minimum or maximum possible value, instead of overflowing
           (or trapping), the keyword `saturated' is optional, `==?',
           `==?satu' and `==?saturated' are therefore identical.  Available
           operators are `-lt' (less than), `-le' (less than or equal to),
           `-eq' (equal), `-ne' (not equal), `-ge' (greater than or equal to),
           and `-gt' (greater than).
           String and regular expression data operators compare the left and
           right hand side according to their textual content.  Unset vari-
           ables are treated as the empty string.  Via the question mark `?'
           modifier suffix a case-insensitive operation mode is available, the
           keyword `case' is optional, `==?' and `==?case' are identical.
           Available string operators are `<' (less than), `<=' (less than or
           equal to), `==' (equal), `!=' (not equal), `>=' (greater than or
           equal to), `>' (greater than), `=%' (is substring of) and `!%' (is
           not substring of).  By default these operators work on bytes and
           (therefore) do not take into account character set specifics.  If
           the case-insensitivity modifier has been used, case is ignored
           according to the rules of the US-ASCII encoding, i.e., bytes are
           still compared.
           When the [Option]al regular expression support is available, the
           additional string operators `=~' and `!~' can be used.  They treat
           the right hand side as an extended regular expression that is
           matched according to the active locale (see Character sets), i.e.,
           character sets should be honoured correctly.
           Conditions can be joined via AND-OR lists (where the AND operator
           is `&&' and the OR operator is `||'), which have equal precedence
           and will be evaluated with left associativity, thus using the same
           syntax that is known for the sh(1).  It is also possible to form
           groups of conditions and lists by enclosing them in pairs of brack-
           ets `[ ... ]', which may be interlocked within each other, and also
           be joined via AND-OR lists.
           The results of individual conditions and entire groups may be modi-
           fied via unary operators: the unary operator `!' will reverse the
           result.
                 wysh set v15-compat=yes # with value: automatic "wysh"!
                 if -N debug;echo *debug* set;else;echo not;endif
                 if "$ttycharset" == UTF-8 || "$ttycharset" ==?cas UTF8
                   echo ttycharset is UTF-8, the former case-sensitive!
                 endif
                 set t1=one t2=one
                 if [ "${t1}" == "${t2}" ]
                   echo These two variables are equal
                 endif
                 if "$features" =% ,+regex, && "$TERM" =~?case ^xterm.*
                   echo ..in an X terminal
                 endif
                 if [ [ true ] && [ [ "${debug}" != '' ] || \
                     [ "$verbose" != '' ] ] ]
                   echo Noisy, noisy
                 endif
                 if true && [ -n "$debug" || -n "${verbose}" ]
                   echo Left associativity, as is known from the shell
                 endif
     ignore
           (ig) Identical to discard.  Superseded by the multiplexer
           headerpick.
     list  Shows the names of all available commands, in command lookup order.
           [Option] In conjunction with a set variable verbose additional
           information will be provided for each command: the argument type
           will be indicated, the documentation string will be shown, and the
           set of command flags will show up:
           ``local''    command supports the command modifier local.
           ``vput''     command supports the command modifier vput.
           `*!*'        the error number is tracked in !.
           `needs-box'  whether the command needs an active mailbox, a folder.
           `ok:'        indicators whether command is ...
                        `batch/interactive'
                                      usable in interactive or batch mode
                                      (-#).
                        `send-mode'   usable in send mode.
                        `subprocess'  allowed to be used when running in a
                                      subprocess instance, for example from
                                      within a macro that is called via
                                      on-compose-splice.
           `not ok:'    indicators whether command is not ...
                        `compose mode'  available in Compose mode.
                        `startup'       available during program startup, like
                                        in Resource files.
           `gabby'      The command produces history-gabby history entries.
     localopts
           Enforce change localization of environ (linked) ENVIRONMENT as well
           as (global) INTERNAL VARIABLES, meaning that their state will be
           reverted to the former one once the ``covered scope'' is left.
           Just like the command modifier local, which provides block-scope
           localization for some commands (instead), it can only be used
           inside of macro definition blocks introduced by account or define.
           The covered scope of an account is left once a different account is
           activated, and some macros, notably folder-hooks, use their own
           specific notion of covered scope, here it will be extended until
           the folder is left again.
           This setting stacks up: i.e., if `macro1' enables change localiza-
           tion and calls `macro2', which explicitly resets localization, then
           any value changes within `macro2' will still be reverted when the
           scope of `macro1' is left.  (Caveats: if in this example `macro2'
           changes to a different account which sets some variables that are
           already covered by localizations, their scope will be extended, and
           in fact leaving the account will (thus) restore settings in
           (likely) global scope which actually were defined in a local, macro
           private context!)
           This command takes one or two arguments, the optional first one
           specifies an attribute that may be one of scope, which refers to
           the current scope and is thus the default, call, which causes any
           macro that is being called to be started with localization enabled
           by default, as well as call-fixate, which (if enabled) disallows
           any called macro to turn off localization: like this it can be
           ensured that once the current scope regains control, any changes
           made in deeper levels have been reverted.  The latter two are mutu-
           ally exclusive, and neither affects xcall.  The (second) argument
           is interpreted as a boolean (string, see INTERNAL VARIABLES) and
           states whether the given attribute shall be turned on or off.
                 define temporary_settings {
                   set possibly_global_option1
                   localopts on
                   set localized_option1
                   set localized_option2
                   localopts scope off
                   set possibly_global_option2
                 }
     Lfollowup, Lreply
           (Compose mode) Reply to messages that come in via known (mlist) or
           subscribed (mlsubscribe) mailing lists, or pretend to do so (see
           Mailing lists): on top of the usual followup and reply, respec-
           tively, functionality this will actively resort and even remove
           message recipients in order to generate a message that is supposed
           to be sent to a mailing list.  For example it will also implicitly
           generate a `Mail-Followup-To:' header if that seems useful, regard-
           less of the setting of the variable followup-to.  For more documen-
           tation please refer to On sending mail, and non-interactive mode.
           This may generate the errors ^ERR-DESTADDRREQ if no receiver has
           been specified, ^ERR-PERM if some addressees where rejected by
           expandaddr, ^ERR-IO if an I/O error occurs, ^ERR-NOTSUP if a neces-
           sary character set conversion fails, and ^ERR-INVAL for other
           errors.  It can also fail with errors of Specifying messages.
           Occurrence of some of the errors depend on the value of expandaddr.
           Any error stops processing of further messages.
     Mail  (Compose mode) Similar to mail, but saves the message in a file
           named after the local part of the first recipient's address
           (instead of in record).
     mail  (Compose mode)(m) Takes a (list of) recipient address(es) as (an)
           argument(s), or asks on standard input if none were given; then
           collects the remaining mail content and sends it out.  Unless the
           internal variable fullnames is set recipient addresses will be
           stripped from comments, names etc.  For more documentation please
           refer to On sending mail, and non-interactive mode.
           This may generate the errors ^ERR-DESTADDRREQ if no receiver has
           been specified, ^ERR-PERM if some addressees where rejected by
           expandaddr, ^ERR-NOTSUP if multiple messages have been specified,
           ^ERR-IO if an I/O error occurs, ^ERR-NOTSUP if a necessary charac-
           ter set conversion fails, and ^ERR-INVAL for other errors.  It can
           also fail with errors of Specifying messages.  Occurrence of some
           of the errors depend on the value of expandaddr.
     mailcap
           [Option] When used without arguments or if show has been given the
           content of The Mailcap files cache is shown, (re-)initializing it
           first (as necessary.  If the argument is load then the cache will
           only be (re-)initialized, and clear will remove its contents.  Note
           that S-nail will try to load the files only once, use `mailcap
           clear' to unlock further attempts.  Loading and parsing can be made
           more verbose.
     mbox  (mb) The given message list is to be sent to the secondary mailbox
           MBOX when S-nail is quit; this is the default action unless the
           variable hold is set.  [v15 behaviour may differ] This command can
           only be used in a primary system mailbox.
     mimetype, unmimetype
           [Only new quoting rules] Without arguments the content of the MIME
           type cache will displayed; a more verbose listing will be produced
           if either of debug or verbose are set.  When given arguments they
           will be joined, interpreted as shown in The mime.types files (also
           see HTML mail and MIME attachments), and the resulting entry will
           be added (prepended) to the cache.  In any event MIME type sources
           are loaded first as necessary - mimetypes-load-control can be used
           to fine-tune which sources are actually loaded.
           The latter command deletes all specifications of the given MIME
           type, thus `? unmimetype text/plain' will remove all registered
           specifications for the MIME type `text/plain'.  The special name
           `*' will discard all existing MIME types, just as will `reset', but
           which also reenables cache initialization via
           mimetypes-load-control.
     mimeview
           [v15 behaviour may differ] Only available in interactive mode, this
           command allows execution of external MIME type handlers which do
           not integrate into the normal type output (see HTML mail and MIME
           attachments).  ([v15 behaviour may differ] No syntax to directly
           address parts, this restriction may vanish.)  The user will be
           asked for each non-text part of the given message in turn whether
           the registered handler shall be used to display the part.
     mlist, unmlist
           [Only new quoting rules] Manage the list of known Mailing lists;
           subscriptions are controlled via mlsubscribe.  The latter command
           deletes all given arguments, or all at once when given the asterisk
           `*'.  The former shows the list of all currently known lists if
           used without arguments, otherwise the given arguments will become
           known.  [Option] In the latter case, arguments which contain any of
           the magic regular expression characters will be interpreted as one,
           possibly matching many addresses; these will be sequentially
           matched via linked lists instead of being looked up in a dictio-
           nary.
     mlsubscribe, unmlsubscribe
           Building upon the command pair mlist, unmlist, but only managing
           the subscription attribute of mailing lists.  (The former will also
           create not yet existing mailing lists.)
     Move  Similar to move, but move the messages to a file named after the
           local part of the sender of the first message instead of taking a
           filename argument; outfolder is inspected to decide on the actual
           storage location.
     move  Acts like copy but marks the messages for deletion if they were
           transferred successfully.
     More  Like more, but also displays header fields which would not pass the
           headerpick selection, and all MIME parts.  Identical to Page.
     more  Invokes the PAGER on the given messages, even in non-interactive
           mode and as long as the standard output is a terminal.  Identical
           to page.
     mtaaliases
           [Option] When used without arguments or if show has been given the
           content of the mta-aliases cache is shown, (re-)initializing it
           first (as necessary).  If the argument is load then the cache will
           only be (re-)initialized, and clear will remove its contents.
     netrc
           [Option] When used without arguments, or when the argument was show
           the content of the ~/.netrc cache is shown, initializing it as nec-
           essary.  If the argument is load then the cache will be (re)loaded,
           whereas clear removes it.  Loading and parsing can be made more
           verbose.  lookup will query the cache for the URL given as the sec-
           ond argument (`[USER@]HOST').  See netrc-lookup, netrc-pipe and the
           section On URL syntax and credential lookup; the section The .netrc
           file documents the file format in detail.
     newmail
           Checks for new mail in the current folder without committing any
           changes before.  If new mail is present, a message is shown.  If
           the header variable is set, the headers of each new message are
           also shown.  This command is not available for all mailbox types.
     next  (n) (like `+' or ``ENTER'') Goes to the next message in sequence
           and types it.  With an argument list, types the next matching mes-
           sage.
     New   Same as Unread.
     new   Same as unread.
     noop  If the current folder is accessed via a network connection, a
           ``NOOP'' command is sent, otherwise no operation is performed.
     Page  Like page, but also displays header fields which would not pass the
           headerpick selection, and all MIME parts.  Identical to More.
     page  Invokes the PAGER on the given messages, even in non-interactive
           mode and as long as the standard output is a terminal.  Identical
           to more.
     Pipe  Like pipe but also pipes header fields which would not pass the
           headerpick selection, and all parts of MIME `multipart/alternative'
           messages.
     pipe  (pi) Takes an optional message list and shell command (that
           defaults to cmd), and pipes the messages through the command.  If
           the page variable is set, every message is followed by a formfeed
           character.
     preserve
           (pre) A synonym for hold.
     Print
           (P) Alias for Type.
     print
           (p) Research UNIX equivalent of type.
     quit  (q) Terminates the session, saving all undeleted, unsaved messages
           in the current secondary mailbox MBOX, preserving all messages
           marked with hold or preserve or never referenced in the system
           inbox, and removing all other messages from the primary system
           mailbox.  If new mail has arrived during the session, the message
           ``You have new mail'' will be shown.  If given while editing a
           mailbox file with the command line option -f, then the edit file is
           rewritten.  A return to the shell is effected, unless the rewrite
           of edit file fails, in which case the user can escape with the exit
           command.  The optional status number argument will be passed
           through to exit(3).  [v15 behaviour may differ] For now it can hap-
           pen that the given status will be overwritten, later this will only
           occur if a later error needs to be reported onto an otherwise suc-
           cess indicating status.
     read  [Only new quoting rules] Read a line from standard input, or the
           channel set active via readctl, and assign the data, which will be
           split as indicated by ifs, to the given variables.  The variable
           names are checked by the same rules as documented for vput, and the
           same error codes will be seen in !; the exit status ? indicates the
           number of bytes read, it will be `-1' with the error number ! set
           to ^ERR-BADF in case of I/O errors, or ^ERR-NONE upon End-Of-File.
           If there are more fields than variables, assigns successive fields
           to the last given variable.  If there are less fields than vari-
           ables, assigns the empty string to the remains.
                 ? read a b c
                    H  e  l  l  o
                 ? echo "<$a> <$b> <$c>"
                 <H> <e> <l  l  o>
                 ? wysh set ifs=:; read a b c;unset ifs
                 hey2.0,:"'you    ",:world!:mars.:
                 ? echo $?/$^ERRNAME / <$a><$b><$c>
                 0/NONE / <hey2.0,><"'you    ",><world!:mars.:><><>
     readsh
           [Only new quoting rules] Like read, but splits on shell token
           boundaries (see Shell-style argument quoting) rather than at ifs.
           [v15 behaviour may differ] Could become a commandalias, maybe `read
           --tokenize --'.
     readall
           [Only new quoting rules] Read anything from standard input, or the
           channel set active via readctl, and assign the data to the given
           variable.  The variable name is checked by the same rules as docu-
           mented for vput, and the same error codes will be seen in !; the
           exit status ? indicates the number of bytes read, it will be `-1'
           with the error number ! set to ^ERR-BADF in case of I/O errors, or
           ^ERR-NONE upon End-Of-File.  [v15 behaviour may differ] The input
           data length is restricted to 31-bits.
     readctl
           [Only new quoting rules] Manages input channels for read, readsh
           and readall, to be used to avoid complicated or impracticable code,
           like calling read from within a macro in non-interactive mode.
           Without arguments, or when the first argument is show, a listing of
           all known channels is printed.  Channels can otherwise be created,
           and existing channels can be set active and removed by giving the
           string used for creation.
           The channel name is expected to be a file descriptor number, or, if
           parsing the numeric fails, an input file name that undergoes
           Filename transformations.  For example (this example requires a
           modern shell):
                 $ printf 'echon "hey, "\nread a\nyou\necho $a' |\
                   s-nail -R#
                 hey, you
                 $ LC_ALL=C printf 'echon "hey, "\nread a\necho $a' |\
                   LC_ALL=C 6<<< 'you' s-nail -R#X'readctl create 6'
                 hey, you
     remove
           [Only new quoting rules] Removes the named files or directories.
           If a name refers to a mailbox, say a Maildir mailbox, then a mail-
           box type specific removal will be performed, deleting the complete
           mailbox.  In interactive mode the user is asked for confirmation.
     rename
           [Only new quoting rules] Takes the name of an existing folder and
           the name for the new folder and renames the first to the second
           one.  Filename transformations including shell pathname wildcard
           pattern expansions (glob(7)) are performed on both arguments.  Both
           folders must be of the same type.
     Reply, Respond
           (Compose mode)(R) Identical to reply except that it replies to only
           the sender of each message of the given list, by using the first
           message as the template to quote, for the `Subject:' etc.; setting
           flipr will exchange this command with reply.
     reply, respond
           (Compose mode)(r) Take a message (list) and group-respond (to each
           in turn) by addressing the sender and all recipients, subject to
           fullnames and alternates processing.  followup-to,
           followup-to-honour, reply-to-honour as well as recipients-in-cc
           influence response behaviour.  quote as well as quote-as-attachment
           configure whether responded-to message shall be quoted etc.,
           content-description-quote-attachment may be used.  Setting flipr
           will exchange this command with Reply.  The command Lreply offers
           special support for replying to mailing lists.  For more documenta-
           tion please refer to On sending mail, and non-interactive mode.
           This may generate the errors ^ERR-DESTADDRREQ if no receiver has
           been specified, or was rejected by expandaddr policy, ^ERR-IO if an
           I/O error occurs, ^ERR-NOTSUP if a necessary character set conver-
           sion fails, and ^ERR-INVAL for other errors.  It can also fail with
           errors of Specifying messages.  Any error stops processing of fur-
           ther messages.
     Resend
           Like resend, but does not add any header lines.  This is not a way
           to hide the sender's identity, but useful for sending a message
           again to the same recipients.
     resend
           Takes a list of messages and a name, and sends each message to the
           given addressee, which is subject to fullnames.  `Resent-From:' and
           related header fields are prepended to the new copy of the message.
           Saving in record is only performed if record-resent is set.  [v15
           behaviour may differ](Compose mode) is not entered, the only sup-
           ported hooks are on-resend-enter and on-resend-cleanup.
           This may generate the errors ^ERR-DESTADDRREQ if no receiver has
           been specified, or was rejected by expandaddr policy, ^ERR-IO if an
           I/O error occurs, ^ERR-NOTSUP if a necessary character set conver-
           sion fails, and ^ERR-INVAL for other errors.  It can also fail with
           errors of Specifying messages.  Any error stops processing of fur-
           ther messages.
     retain
           (ret) Superseded by the multiplexer headerpick.
     return
           Only available inside of a defined macro or an account, this com-
           mand returns control of execution to the outer scope.  The two
           optional parameters are positive decimal numbers and default to 0:
           the first specifies the 32-bit return value (stored in ? [v15 be-
           haviour may differ] and later extended to 64-bit), the second the
           32-bit error number (stored in !).  As documented for ? a non-0
           exit status may cause the program to exit.
     Save  (S) Similar to save, but saves the messages in a file named after
           the local part of the sender of the first message instead of taking
           a filename argument; outfolder is inspected to decide on the actual
           storage location.
     save  (s) Takes a message list and a filename and appends each message in
           turn to the end of the file.  Filename transformations including
           shell pathname wildcard pattern expansions (glob(7)) is performed
           on the filename.  If no filename is given, the secondary mailbox
           MBOX is used.  The filename in quotes, followed by the generated
           character count is echoed on the user's terminal.  If editing a
           primary system mailbox the messages are marked for deletion.  To
           filter the saved header fields to the desired subset use the `save'
           slot of the white- and blacklisting command headerpick.  Also see
           Copy.
     savediscard
           [Obsolete] Superseded by the multiplexer headerpick.
     saveignore
           [Obsolete] Superseded by the multiplexer headerpick.
     saveretain
           [Obsolete] Superseded by the multiplexer headerpick.
     search
           Takes a message specification (list) and displays a header summary
           of all matching messages, as via headers.  This command is an alias
           of from.  Also see Specifying messages.
     seen  Takes a message list and marks all messages as having been read.
     set, unset
           (se, [Only new quoting rules] uns) The latter command will delete
           all given global variables, or only block-scope local ones if the
           local command modifier has been used.  The former, when used with-
           out arguments, will show all currently known variables, being more
           verbose if either of debug or verbose is set.  Remarks: this list
           mode will not automatically link-in (known) ENVIRONMENT variables,
           this only happens for explicit addressing, examples are varshow,
           using a variable in an if condition or a string passed to echo,
           explicit setting, as well as some program-internal use cases (look-
           ups).
           Otherwise the given variables (and arguments) are set or adjusted.
           Arguments are of the form `name=value' (no space before or after
           `='), or plain `name' if there is no value, i.e., a boolean vari-
           able.  If a name begins with `no', as in `set nosave', the effect
           is the same as invoking the unset command with the remaining part
           of the variable (`unset save').  [v15 behaviour may differ] In con-
           junction with the wysh (or local) command prefix(es) Shell-style
           argument quoting can be used to quote arguments as necessary.  [v15
           behaviour may differ] Otherwise quotation marks may be placed
           around any part of the assignment statement to quote blanks or
           tabs.
           When operating in global scope any `name' that is known to map to
           an environment variable will automatically cause updates in the
           program environment (unsetting a variable in the environment
           requires corresponding system support) -- use the command environ
           for further environmental control.  If the command modifier local
           has been used to enforce local scoping then the given user vari-
           ables will be garbage collected when the local scope is left; for
           INTERNAL VARIABLES, however, local behaves the same as if localopts
           would have been set (temporarily), which means that changes are
           inherited by deeper scopes.  Also see varshow and the sections
           INTERNAL VARIABLES and ENVIRONMENT.
                 ? wysh set indentprefix=' -> '
                 ? wysh set atab=$'' aspace=' ' zero=0
     shcodec
           Apply shell quoting rules to the given raw-data arguments.  Sup-
           ports vput (see Command modifiers).  The first argument specifies
           the operation: [+]e[ncode] or d[ecode] cause shell quoting to be
           applied to the remains of the line, and expanded away thereof,
           respectively.  If the former is prefixed with a plus-sign, the
           quoted result will not be roundtrip enabled, and thus can be
           decoded only in the very same environment that was used to perform
           the encode; also see mle-quote-rndtrip.  If the coding operation
           fails the error number ! is set to ^ERR-CANCELED, and the unmodi-
           fied input is used as the result; the error number may change again
           due to output or result storage errors.
     shell
           [Only new quoting rules] (sh) Invokes an interactive version of the
           shell, and returns its exit status.
     shortcut, unshortcut
           [Only new quoting rules] Manage the file- or pathname shortcuts as
           documented for folder.  The latter command deletes all shortcuts
           given as arguments, or all at once when given the asterisk `*'.
           The former shows the list of all currently defined shortcuts if
           used without arguments, the target of the given with a single argu-
           ment.  Otherwise arguments are treated as pairs of shortcuts and
           their desired expansion, creating new or updating already existing
           ones.
     shift
           [Only new quoting rules] Shift the positional parameter stack
           (starting at 1) by the given number (which must be a positive deci-
           mal), or 1 if no argument has been given.  It is an error if the
           value exceeds the number of positional parameters.  If the given
           number is 0, no action is performed, successfully.  The stack as
           such can be managed via vpospar.  Note this command will fail in
           account and hook macros unless the positional parameter stack has
           been explicitly created in the current context via vpospar.
     show  Like type, but performs neither MIME decoding nor decryption, so
           that the raw message text is shown.
     size  (si) Shows the size in characters of each message of the given mes-
           sage list.
     sleep
           [Only new quoting rules] Sleep for the specified number of seconds
           (and optionally milliseconds), by default interruptible.  If a
           third argument is given the sleep will be uninterruptible, other-
           wise the error number ! will be set to ^ERR-INTR if the sleep has
           been interrupted.  The command will fail and the error number will
           be ^ERR-OVERFLOW if the given duration(s) overflow the time
           datatype, and ^ERR-INVAL if the given durations are no valid inte-
           gers.
     sort, unsort
           The latter command disables sorted or threaded mode, returns to
           normal message order and, if the header variable is set, displays a
           header summary.  The former command shows the current sorting cri-
           terion when used without an argument, but creates a sorted repre-
           sentation of the current folder otherwise, and changes the next
           command and the addressing modes such that they refer to messages
           in the sorted order.  Message numbers are the same as in regular
           mode.  If the header variable is set, a header summary in the new
           order is also displayed.  Automatic folder sorting can be enabled
           by setting the autosort variable, as in `set autosort=thread'.
           Possible sorting criterions are:
           date     Sort the messages by their `Date:' field, that is by the
                    time they were sent.
           from     Sort messages by the value of their `From:' field, that is
                    by the address of the sender.  If the showname variable is
                    set, the sender's real name (if any) is used.
           size     Sort the messages by their size.
           spam     [Option] Sort the message by their spam score, as has been
                    classified by spamrate.
           status   Sort the messages by their message status.
           subject  Sort the messages by their subject.
           thread   Create a threaded display.
           to       Sort messages by the value of their `To:' field, that is
                    by the address of the recipient.  If the showname variable
                    is set, the recipient's real name (if any) is used.
     source
           [Only new quoting rules] (so) The source command reads commands
           from the given file.  Filename transformations will be applied.  If
           the given expanded argument ends with a vertical bar `|' then the
           argument will instead be interpreted as a shell command and S-nail
           will read the output generated by it.  Dependent on the settings of
           posix and errexit, and also dependent on whether the command modi-
           fier ignerr had been used, encountering errors will stop sourcing
           of the given input.  [v15 behaviour may differ] Note that source
           cannot be used from within macros that execute as folder-hooks or
           accounts, i.e., it can only be called from macros that were called.
     source_if
           [Only new quoting rules] The difference to source (beside not sup-
           porting pipe syntax aka shell command input) is that this command
           will not generate an error nor warn if the given file argument can-
           not be opened successfully.
     spamclear
           [Option] Takes a list of messages and clears their `is-spam' flag.
     spamforget
           [Option] Takes a list of messages and causes the spam-interface to
           forget it has ever used them to train its Bayesian filter.  Unless
           otherwise noted the `is-spam' flag of the message is inspected to
           chose whether a message shall be forgotten to be ``ham'' or
           ``spam''.
     spamham
           [Option] Takes a list of messages and informs the Bayesian filter
           of the spam-interface that they are ``ham''.  This also clears the
           `is-spam' flag of the messages in question.
     spamrate
           [Option] Takes a list of messages and rates them using the config-
           ured spam-interface, without modifying the messages, but setting
           their `is-spam' flag as appropriate; because the spam rating head-
           ers are lost the rate will be forgotten once the mailbox is left.
           Refer to the manual section Handling spam for the complete picture
           of spam handling in S-nail.
     spamset
           [Option] Takes a list of messages and sets their `is-spam' flag.
     spamspam
           [Option] Takes a list of messages and informs the Bayesian filter
           of the spam-interface that they are ``spam''.  This also sets the
           `is-spam' flag of the messages in question.
     thread
           [Obsolete] The same as `sort thread' (consider using a
           `commandalias' as necessary).
     tls   [Only new quoting rules] TLS information and management command
           multiplexer to aid in Encrypted network communication, mostly
           available only if the term `,+sockets,' is included in features.
           Commands support vput if so documented (see Command modifiers).
           The result that is shown in case of errors is always the empty
           string, errors can be identified via the error number !.  For exam-
           ple, string length overflows are caught and set ! to ^ERR-OVERFLOW.
           The TLS configuration is honoured, especially tls-verify.
                 ? vput tls result fingerprint pop3s://ex.am.ple
                 ? echo $?/$!/$^ERRNAME: $result
           certchain Show the complete verified peer certificate chain.
                     Includes informational fields in conjunction with
                     verbose.
           certificate Show only the peer certificate, without any signers.
                     Includes informational fields in conjunction with
                     verbose.
           fingerprint Show the tls-fingerprint-digested fingerprint of the
                     certificate of the given HOST (`server:port', where the
                     port defaults to the HTTPS port, 443).  tls-fingerprint
                     is actively ignored for the runtime of this command.
     Top   Like top but always uses the headerpick `type' slot for white- and
           blacklisting header fields.
     top   (to) Takes a message list and types out the first toplines lines of
           each message on the user's terminal.  Unless a special selection
           has been established for the `top' slot of the headerpick command,
           the only header fields that are displayed are `From:', `To:',
           `Cc:', and `Subject:'.  Top will always use the `type' headerpick
           selection instead.  It is possible to apply compression to what is
           displayed by setting topsqueeze.  Messages are decrypted and con-
           verted to the terminal character set if necessary.
     touch
           (tou) Takes a message list and marks the messages for saving in the
           secondary mailbox MBOX.  S-nail deviates from the POSIX standard
           with this command, as a following next command will display the
           following message instead of the current one.
     Type  (T) Like type but also displays header fields which would not pass
           the headerpick selection, and all visualizable parts of MIME
           `multipart/alternative' messages.
     type  (t) Takes a message list and types out each message on the user's
           terminal.  The display of message headers is selectable via
           headerpick.  For MIME multipart messages, all parts with a content
           type of `text', all parts which have a registered MIME type handler
           (see HTML mail and MIME attachments) which produces plain text out-
           put, and all `message' parts are shown, others are hidden except
           for their headers.  Messages are decrypted and converted to the
           terminal character set if necessary.  The command mimeview can be
           used to display parts which are not displayable as plain text.
     unaccount
           See account.
     unalias
           (una) See alias.
     unanswered
           See answered.
     unbind
           See bind.
     uncollapse
           See collapse.
     uncolour
           See colour.
     undefine
           See define.
     undelete
           See delete.
     undraft
           See draft.
     unflag
           See flag.
     unfwdignore
           [Obsolete] Superseded by the multiplexer headerpick.
     unfwdretain
           [Obsolete] Superseded by the multiplexer headerpick.
     unignore
           Superseded by the multiplexer headerpick.
     unmimetype
           See mimetype.
     unmlist
           See mlist.
     unmlsubscribe
           See mlsubscribe.
     Unread
           Same as unread.
     unread
           Takes a message list and marks each message as not having been
           read.
     unretain
           Superseded by the multiplexer headerpick.
     unsaveignore
           [Obsolete] Superseded by the multiplexer headerpick.
     unsaveretain
           [Obsolete] Superseded by the multiplexer headerpick.
     unset
           [Only new quoting rules] (uns) See set.
     unshortcut
           See shortcut.
     unsort
           See short.
     unthread
           [Obsolete] Same as unsort.
     urlcodec
           Perform URL percent codec operations on the raw-data argument,
           rather according to RFC 3986.  The first argument specifies the
           operation: e[ncode] or d[ecode] perform plain URL percent en- and
           decoding, respectively.  p[ath]enc[ode] and p[ath]dec[ode] perform
           a slightly modified operation which should be better for pathnames:
           it does not allow a tilde `~', and will neither accept hyphen-minus
           `-' nor dot `'.  as an initial character.  The remains of the line
           form the URL data which is to be converted.  This is a character
           set agnostic operation, and it may thus decode bytes which are
           invalid in the current ttycharset.
           Supports vput (see Command modifiers), and manages the error number
           !.  If the coding operation fails the error number ! is set to
           ^ERR-CANCELED, and the unmodified input is used as the result; the
           error number may change again due to output or result storage
           errors.  [v15 behaviour may differ] This command does not know
           about URLs beside what is documented.  (vexpr offers a makeprint
           subcommand, shall the URL be displayed.)
     varshow
           [Only new quoting rules] This command produces the same output as
           the listing mode of set, including verboseity adjustments, but only
           for the given variables.
     verify
           [Option] Takes a message list and verifies each message.  If a mes-
           sage is not a S/MIME signed message, verification will fail for it.
           The verification process checks if the message was signed using a
           valid certificate, if the message sender's email address matches
           one of those contained within the certificate, and if the message
           content has been altered.
     version
           Shows the version and features of S-nail, optionally in a more
           verbose form which also includes the build and running system envi-
           ronment.  This command supports vput (see Command modifiers).
     vexpr
           [Only new quoting rules] A multiplexer command which offers signed
           64-bit numeric calculations, as well as other, mostly string-based
           operations.  C-style byte string operations are available via csop.
           The first argument defines the number, type, and meaning of the
           remaining arguments.  An empty number argument is treated as 0.
           Supports vput (see Command modifiers).  The result shown in case of
           errors is `-1' for usage errors and numeric operations, the empty
           string otherwise; ``soft'' errors, like when a search operation
           failed, will also set the ! error number to ^ERR-NODATA.  Except
           when otherwise noted numeric arguments are parsed as signed 64-bit
           numbers, and errors will be reported in the error number ! as the
           numeric error ^ERR-RANGE.
           Numeric operations work on one or two signed 64-bit integers.  Num-
           bers prefixed with `0x' or `0X' are interpreted as hexadecimal
           (base 16) numbers, whereas `0' indicates octal (base 8), and `0b'
           as well as `0B' denote binary (base 2) numbers.  It is possible to
           use any base in between 2 and 36, inclusive, with the `BASE#number'
           notation, where the base is given as an unsigned decimal number, so
           `16#AFFE' is a different way of specifying a hexadecimal number.
           Unsigned interpretation of a number can be enforced by prefixing an
           `u' (case-insensitively), as in `u-110'; this is not necessary for
           power-of-two bases (2, 4, 8, 16 and 32), which will be interpreted
           as unsigned by default, but it still makes a difference regarding
           overflow detection and overflow constant.  It is possible to
           enforce signed interpretation by (instead) prefixing a `s' (case-
           insensitively).  The number sign notation uses a permissive parse
           mode and as such supports complicated conditions out of the box:
                 ? wysh set ifs=:;read i;unset ifs;echo $i;vexpr pb 2 10#$i
                    -009
                 <   -009>
                 0b1001
           One integer is expected by assignment (equals sign `='), which does
           nothing but parsing the argument, thus detecting validity and pos-
           sible overflow conditions, unary not (tilde `~'), which creates the
           bitwise complement, and unary plus and minus.  Two integers are
           used by addition (plus sign `+'), subtraction (hyphen-minus `-'),
           multiplication (asterisk `*'), division (solidus `/') and modulo
           (percent sign `%'), as well as for the bitwise operators logical or
           (vertical bar `|', to be quoted) , bitwise and (ampersand `&', to
           be quoted) , bitwise xor (circumflex `^'), the bitwise signed left-
           and right shifts (`<<', `>>'), as well as for the unsigned right
           shift `>>>'.
           Another numeric operation is pbase, which takes a number base in
           between 2 and 36, inclusive, and will act on the second number
           given just the same as what equals sign `=' does, but the number
           result will be formatted in the base given, as a signed 64-bit num-
           ber unless unsigned interpretation of the input number had been
           forced (with an u prefix).
           Numeric operations support a saturated mode via the question mark
           `?' modifier suffix; the keyword `saturated' is optional, `+?',
           `+?satu', and `+?saturated' are therefore identical.  In saturated
           mode overflow errors and division and modulo by zero are no longer
           reported via the exit status, but the result will linger at the
           minimum or maximum possible value, instead of overflowing (or trap-
           ping).  This is true also for the argument parse step.  For the
           bitwise shifts, the saturated maximum is 63.  Any caught overflow
           will be reported via the error number ! as ^ERR-OVERFLOW.
                 ? vput vexpr res -? +1 -9223372036854775808
                 ? echo $?/$!/$^ERRNAME:$res
                 0/75/OVERFLOW:-9223372036854775808
           Character set agnostic string functions have no notion of locale
           settings and character sets.
           date-utc  Outputs the current date and time in UTC (Coordinated
                     Universal Time) with values named such that `vput vexpr x
                     date-utc; eval wysh set $x' creates accessible variables.
           date-stamp-utc Outputs a RFC 3339 internet date/time format of UTC.
           epoch     The seconds and nanoseconds since the Unix epoch
                     (1970-01-01T00:00:00) named `epoch_sec' and `epoch_nsec'
                     such that `vput vexpr x epoch; eval wysh set $x' creates
                     accessible variables.
           file-expand Performs the usual Filename transformations on its
                     argument.
           file-stat, file-lstat Perform the usual Filename transformations on
                     the argument, then call stat(2) and lstat(2), respec-
                     tively, and output values such that `vput vexpr x
                     file-stat FILE; eval wysh set $x' creates accessible
                     variables.  The variable `st_type' uses solidus `/' to
                     denote directories, commercial at `@' for links, number
                     sign `#' for block devices, percent sign `%' for for
                     character devices, vertical bar `|' for FIFOs, equal sign
                     `=' for sockets, and the period `.' for the rest.
           random    Generates a random string of the given length, or of
                     PATH_MAX bytes (a constant from /usr/include) if the
                     value 0 is given; the random string will be base64url
                     encoded according to RFC 4648, and thus be usable as a
                     (portable) filename.
           String operations work, sufficient support provided, according to
           the active user's locale encoding and character set (see Character
           sets).  Where the question mark `?' modifier suffix is supported, a
           case-insensitive operation mode is available; the keyword `case' is
           optional, `regex?' and `regex?case' are therefore identical.
           makeprint (One-way) Converts the argument to something safely
                     printable on the terminal.
           regex     [Option] A string operation that will try to match the
                     first argument with the regular expression given as the
                     second argument.  `?' modifier suffix is supported.  If
                     the optional third argument has been given then instead
                     of showing the match offset a replacement operation is
                     performed: the third argument is treated as if specified
                     within dollar-single-quote (see Shell-style argument
                     quoting), and any occurrence of a positional parameter,
                     for example 0, 1 etc. is replaced with the according
                     match group of the regular expression:
                           ? vput vexpr res regex bananarama \
                               (.*)NanA(.*) '\${1}au\$2'
                           ? echo $?/$!/$^ERRNAME:$res:
                           1/61/NODATA::
                           ? vput vexpr res regex?case bananarama \
                               (.*)NanA(.*) '\${1}uauf\$2'
                           ? echo $?/$!/$^ERRNAME:$res:
                           0/0/NONE:bauauframa:
     vpospar
           [Only new quoting rules] Manage the positional parameter stack (see
           1, #, *, @ as well as shift).  If the first argument is `clear',
           then the positional parameter stack of the current context, or the
           global one, if there is none, is cleared.  If it is `set', then the
           remaining arguments will be used to (re)create the stack, if the
           parameter stack size limit is excessed an ^ERR-OVERFLOW error will
           occur.
           If the first argument is `quote', a round-trip capable representa-
           tion of the stack contents is created, with each quoted parameter
           separated from each other with the first character of ifs, and fol-
           lowed by the first character of if-ws, if that is not empty and not
           identical to the first.  If that results in no separation at all a
           space character is used.  This mode supports vput (see Command
           modifiers).  I.e., the subcommands `set' and `quote' can be used
           (in conjunction with eval) to (re)create an argument stack from and
           to a single variable losslessly.
                 ? vpospar set hey, "'you    ", world!
                 ? echo $#: <${1}><${2}><${3}>
                 ? vput vpospar x quote
                 ? vpospar clear
                 ? echo $#: <${1}><${2}><${3}>
                 ? eval vpospar set ${x}
                 ? echo $#: <${1}><${2}><${3}>
     visual
           (v) Takes a message list and invokes the VISUAL display editor on
           each message.  Modified contents are discarded unless the
           writebackedited variable is set, and are not used unless the mail-
           box can be written to and the editor returns a successful exit sta-
           tus.  edit can be used instead for a less display oriented editor.
     write
           (w) For conventional messages the body without all headers is writ-
           ten.  The original message is never marked for deletion in the
           originating mail folder.  The output is decrypted and converted to
           its native format as necessary.  If the output file exists, the
           text is appended.  If a message is in MIME multipart format its
           first part is written to the specified file as for conventional
           messages, handling of the remains depends on the execution mode.
           No special handling of compressed files is performed.
           In interactive mode the user is consecutively asked for the file-
           names of the processed parts.  For convenience saving of each part
           may be skipped by giving an empty value, the same result as writing
           it to /dev/null.  Shell piping the part content by specifying a
           leading vertical bar `|' character for the filename is supported.
           Other user input undergoes the usual Filename transformations,
           including shell pathname wildcard pattern expansions (glob(7)) and
           shell variable expansion for the message as such, not the individ-
           ual parts, and contents of the destination file are overwritten if
           the file previously existed.  Character set conversion to
           ttycharset is performed when saving text data.
           [v15 behaviour may differ] In non-interactive mode any part which
           does not specify a filename is ignored, and suspicious parts of
           filenames of the remaining parts are URL percent encoded (as via
           urlcodec) to prevent injection of malicious character sequences,
           resulting in a filename that will be written into the current
           directory.  Existing files will not be overwritten, instead the
           part number or a dot are appended after a number sign `#' to the
           name until file creation succeeds (or fails due to other reasons).
     xcall
           [Only new quoting rules] The sole difference to call is that the
           new macro is executed in place of the current one, which will not
           regain control: all resources of the current macro will be released
           first.  This implies that any setting covered by localopts will be
           forgotten and covered variables will become cleaned up.  If this
           command is not used from within a called macro it will silently be
           (a more expensive variant of) call.
     xit   (x) A synonym for exit.
     z     [Only new quoting rules] S-nail presents message headers in
           screenfuls as described under the headers command.  Without argu-
           ments this command scrolls to the next window of messages, likewise
           if the argument is `+'.  An argument of `-' scrolls to the last,
           `^' scrolls to the first, and `$' to the last screen of messages.
           A number argument prefixed by `+' or `-' indicates that the window
           is calculated in relation to the current position, and a number
           without a prefix specifies an absolute position.
     Z     [Only new quoting rules] Similar to z, but scrolls to the next or
           previous window that contains at least one `new' or flagged mes-
           sage.
COMMAND ESCAPES
     Command escapes are available in Compose mode during interactive usage,
     when explicitly requested via -~, and in batch mode (-#).  They perform
     special functions, like editing headers of the message being composed,
     calling normal COMMANDS, yielding a shell, etc.  Command escapes are only
     recognized at the beginning of lines, and consist of an escape followed
     by a command character.  The default escape character is the tilde `~'.
     Unless otherwise documented command escapes ensure proper updates of the
     error number ! and the exit status ?.  The variable errexit controls
     whether a failed operation errors out message compose mode and causes
     program exit.  Escapes may be prefixed by none to multiple single charac-
     ter command modifiers, interspersed whitespace is ignored:
     o   An effect equivalent to the command modifier ignerr can be achieved
         with hyphen-minus `-', overriding errexit.
     o   The modifier dollar `$' evaluates the remains of the line; also see
         Shell-style argument quoting.  [v15 behaviour may differ] For now the
         entire input line is evaluated as a whole; to avoid that control
         operators like semicolon ; are interpreted unintentionally, they must
         be quoted.
     Addition of the command line to the [Option]al history can be prevented
     by placing whitespace directly after escape.  The [Option]al key bindings
     support a compose mode specific context.  The following command escapes
     are supported:
     ~~ string
           Insert the string of text in the message prefaced by a single `~'.
           (If the escape character has been changed, that character must be
           doubled instead.)
     ~! command
           Execute the indicated shell command which follows, replacing
           unescaped exclamation marks with the previously executed command if
           the internal variable bang is set, then return to the message.
     ~.    End compose mode and send the message.  The hooks
           on-compose-splice-shell and on-compose-splice, in order, will be
           called when set, after which, in interactive mode askatend (leading
           to askcc, askbcc) and askattach will be checked as well as asksend,
           after which a set on-compose-leave hook will be called, autocc and
           autobcc will be joined in if set, finally a given
           message-inject-tail will be incorporated, after which the compose
           mode is left.
     ~: S-nail-command or ~_ S-nail-command
           Can be used to execute COMMANDS (which are allowed in compose
           mode).
     ~< filename
           Identical to ~r.
     ~<! command
           command is executed using the shell.  Its standard output is
           inserted into the message.
     ~?    [Option] Write a summary of command escapes.
     ~@ [filename...]
           Append or edit the list of attachments.  Does not manage the error
           number ! and the exit status ? (please use ~^ if error handling is
           necessary).  The append mode expects a list of filename arguments
           as shell tokens (see Shell-style argument quoting; token-separating
           commas are ignored, too), to be interpreted as documented for the
           command line option -a, with the message number exception as below.
           Without filename arguments the attachment list is edited, entry by
           entry; if a filename is left empty, that attachment is deleted from
           the list; once the end of the list is reached either new attach-
           ments may be entered or the session can be quit by committing an
           empty ``new'' attachment.  In non-interactive mode or in batch mode
           (-#) the list of attachments is effectively not edited but instead
           recreated; again, an empty input ends list creation.
           For all modes, if a given filename solely consists of the number
           sign `#' followed by either a valid message number of the currently
           active mailbox, or by a period `.', referring to the current mes-
           sage of the active mailbox, the so-called ``dot'', then the given
           message is attached as a `message/rfc822' MIME message part.  The
           number sign must be quoted to avoid misinterpretation as a shell
           comment character.
     ~| command
           Pipe the message text through the specified filter command.  If the
           command gives no output or terminates abnormally, retain the origi-
           nal text of the message.  The command fmt(1) is often used as a
           rejustifying filter.
           If the first character of the command is a vertical bar, then the
           entire message including header fields is subject to the filter
           command, so `~|| echo Fcc: /tmp/test; cat' will prepend a file-car-
           bon-copy message header.  Also see ~e, ~v.
     ~^ cmd [subcmd [arg3 [arg4]]]
           Inspect and modify the message using the semantics of digmsg,
           therefore arguments are evaluated according to Shell-style argument
           quoting.  Error number ! and exit status ? are not managed: errors
           are handled via the protocol, and hard errors like I/O failures
           cannot be handled.
           The protocol consists of command lines followed by (a) response
           line(s).  The first field of the response line represents a status
           code which specifies whether a command was successful or not,
           whether result data is to be expected, and if, the format of the
           result data.  Response data will be shell quoted as necessary for
           consumption by readsh, or eval and vpospar, to name a few.  Error
           status code lines may optionally contain additional context:
           `210'  Status ok; the remains of the line are the result.
           `211'  Status ok; the rest of the line is optionally used for more
                  status.  What follows are lines of result addresses, termi-
                  nated by an empty line.  All the input, including the empty
                  line, must be consumed before further commands can be
                  issued.  Address lines consist of two token, first the plain
                  network address, e.g., `bob AT exam.ple', followed by the
                  (quoted) full address as known: `'(Lovely) Bob
                  <bob AT exam.ple>''.  Non-network addresses use the first field
                  to indicate the type (hyphen-minus `-' for files, vertical
                  bar `|' for pipes, and number sign `#' for names which will
                  undergo alias processing) instead, the actual value will be
                  in the second field.
           `212'  Status ok; the rest of the line is optionally used for more
                  status.  What follows are lines of furtherly unspecified
                  (quoted) string content, terminated by an empty line.  All
                  the input, including the empty line, must be consumed before
                  further commands can be issued.
           `500'  Syntax error; invalid command.
           `501'  Syntax error or otherwise invalid parameters or arguments.
           `505'  Error: an argument fails verification.  For example an
                  invalid address has been specified (also see expandaddr), or
                  an attempt was made to modify anything in S-nail's own
                  namespace, or a modifying subcommand has been used on a
                  read-only message.
           `506'  Error: an otherwise valid argument is rendered invalid due
                  to context.  For example, a second address is added to a
                  header which may consist of a single address only.
           If a command indicates failure then the message will have remained
           unmodified.  Most commands can fail with `500' if required argu-
           ments are missing, or excessive arguments have been given (false
           command usage).  ([v15 behaviour may differ] The latter does not
           yet occur regularly, because as stated in Shell-style argument
           quoting our argument parser is not yet smart enough to work on sub-
           command base; for example one might get excess argument error for a
           three argument subcommand that receives four arguments, but not for
           a four argument subcommand which receives six arguments: here
           excess will be joined.)  The following (case-insensitive) commands
           are supported:
           attachment This command allows listing, removal and addition of
                    message attachments.  The second argument specifies the
                    subcommand to apply, one of:
                    attribute This uses the same search mechanism as described
                              for remove and prints any known attributes of
                              the first found attachment via `212' upon suc-
                              cess or `501' if no such attachment can be
                              found.  The attributes are written as lines with
                              a keyword and a value token.
                    attribute-at This uses the same search mechanism as
                              described for remove-at and is otherwise identi-
                              cal to attribute.
                    attribute-set This uses the same search mechanism as
                              described for remove, and will set the attribute
                              given as the fourth to the value given as the
                              fifth token argument.  If the value is an empty
                              token, then the given attribute is removed, or
                              reset to a default value if existence of the
                              attribute is crucial.
                              It returns via `210' upon success, with the
                              index of the found attachment following, `505'
                              for message attachments or if the given keyword
                              is invalid, and `501' if no such attachment can
                              be found.  The following keywords may be used
                              (case-insensitively):
                              `filename'  Sets the filename of the MIME part,
                                          i.e., the name that is used for dis-
                                          play and when (suggesting a name
                                          for) saving (purposes).
                              `content-description' Associate some descriptive
                                          information to the attachment's con-
                                          tent, used in favour of the plain
                                          filename by some MUAs.
                              `content-id' May be used for uniquely identify-
                                          ing MIME entities in several con-
                                          texts; this expects a special refer-
                                          ence address format as defined in
                                          RFC 2045 and generates a `505' upon
                                          address content verification fail-
                                          ure.
                              `content-type' Defines the media type/subtype of
                                          the part, which is managed automati-
                                          cally, but can be overwritten.
                              `content-disposition' Automatically set to the
                                          string `attachment'.
                    attribute-set-at This uses the same search mechanism as
                              described for remove-at and is otherwise identi-
                              cal to attribute-set.
                    insert    Adds the attachment given as the third argument,
                              specified exactly as documented for the command
                              line option -a, and supporting the message num-
                              ber extension as documented for ~@.  This
                              reports `210' upon success, with the index of
                              the new attachment following, `505' if the given
                              file cannot be opened, `506' if an on-the-fly
                              performed character set conversion fails, other-
                              wise `501' is reported; this is also reported if
                              character set conversion is requested but not
                              available.
                    list      List all attachments via `212', or report `501'
                              if no attachments exist.  This command is the
                              default command of attachment if no second argu-
                              ment has been given.
                    remove    This will remove the attachment given as the
                              third argument, and report `210' upon success or
                              `501' if no such attachment can be found.  If
                              there exists any path component in the given
                              argument, then an exact match of the path which
                              has been used to create the attachment is used
                              directly, but if only the basename of that path
                              matches then all attachments are traversed to
                              find an exact match first, and the removal
                              occurs afterwards; if multiple basenames match,
                              a `506' error occurs.  Message attachments are
                              treated as absolute pathnames.
                              If no path component exists in the given argu-
                              ment, then all attachments will be searched for
                              `filename=' parameter matches as well as for
                              matches of the basename of the path which has
                              been used when the attachment has been created;
                              multiple matches result in a `506'.
                    remove-at This will interpret the third argument as a num-
                              ber and remove the attachment at that list posi-
                              tion (counting from one!), reporting `210' upon
                              success or `505' if the argument is not a number
                              or `501' if no such attachment exists.
           header   This command allows listing, inspection, and editing of
                    message headers.  Header name case is not normalized, so
                    that case-insensitive comparison should be used when
                    matching names.  The second argument specifies the subcom-
                    mand to apply, one of:
                    insert    Create a new or an additional instance of the
                              header given in the third argument, with the
                              header body content as given in the fourth
                              token.  It may return `501' if the third argu-
                              ment specifies a free-form header field name
                              that is invalid, or if body content extraction
                              fails to succeed, `505' if any extracted address
                              does not pass syntax and/or security checks or
                              on S-nail namespace violations, and `506' to
                              indicate prevention of excessing a single-
                              instance header -- note that `Subject:' can be
                              appended to (a space separator will be added
                              automatically first).  `To:', `Cc:' and `Bcc:'
                              support the `?single' modifier to enforce treat-
                              ment as a single addressee, for example `header
                              insert To?single: 'exa, <m@ple>''; the word
                              `single' is optional.
                              `210' is returned upon success, followed by the
                              name of the header and the list position of the
                              newly inserted instance.  The list position is
                              always 1 for single-instance header fields.  All
                              free-form header fields are managed in a single
                              list; also see customhdr.
                    list      Without a third argument a list of all yet
                              existing headers is given via `210'; this com-
                              mand is the default command of header if no sec-
                              ond argument has been given.  A third argument
                              restricts output to the given header only, which
                              may fail with `501' if no such field is defined.
                    remove    This will remove all instances of the header
                              given as the third argument, reporting `210'
                              upon success, `501' if no such header can be
                              found, and `505' on S-nail namespace violations.
                    remove-at This will remove from the header given as the
                              third argument the instance at the list position
                              (counting from one!) given with the fourth argu-
                              ment, reporting `210' upon success or `505' if
                              the list position argument is not a number or on
                              S-nail namespace violations, and `501' if no
                              such header instance exists.
                    show      Shows the content of the header given as the
                              third argument.  Dependent on the header type
                              this may respond with `211' or `212'; any fail-
                              ure results in `501'.
                    In compose-mode read-only access to optional pseudo head-
                    ers in the S-nail private namespace is available:
                    `Mailx-Command:'
                          The name of the command that generates the message,
                          one of `forward', `Lreply', `mail', `Reply',
                          `reply', `resend'.  This pseudo header always exists
                          (in compose-mode).
                    `Mailx-Raw-To:'
                    `Mailx-Raw-Cc:'
                    `Mailx-Raw-Bcc:'
                          Represent the frozen initial state of these headers
                          before any transformation (alias, alternates,
                          recipients-in-cc etc.) took place.
                    `Mailx-Orig-Sender:'
                    `Mailx-Orig-From:'
                    `Mailx-Orig-To:'
                    `Mailx-Orig-Cc:'
                    `Mailx-Orig-Bcc:'
                          The values of said headers of the original message
                          which has been addressed by any of reply, forward,
                          resend.  The sender field is special as it is filled
                          in with the sole sender according to RFC 5322 rules,
                          it may thus be equal to the from field.
           help, ?  Show an abstract of the above commands via `211'.
           version  This command will print the protocol version via `210'.
     ~A    The same as `~i Sign'.
     ~a    The same as `~i sign'.
     ~b name ...
           Add the given names to the list of blind carbon copy recipients.
     ~c name ...
           Add the given names to the list of carbon copy recipients.
     ~d    Read the file specified by the DEAD variable into the message.
     ~e    Invoke the text EDITOR on the message collected so far, then return
           to compose mode.  ~v can be used for a more display oriented edi-
           tor, and ~|| offers a pipe-based editing approach.
     ~F messages
           Read the named messages into the message being sent, including all
           message headers and MIME parts, and honouring forward-add-cc as
           well as forward-inject-head and forward-inject-tail.  If no mes-
           sages are specified, read in the current message, the ``dot''.
     ~f messages
           Read the named messages into the message being sent.  If no mes-
           sages are specified, read in the current message, the ``dot''.
           Strips down the list of header fields according to the `forward'
           (with posix: `type') white- and blacklist selection of headerpick,
           and honours forward-add-cc as well as forward-inject-head and
           forward-inject-tail.  For MIME multipart messages, only the first
           displayable part is included.
     ~H    In interactive mode, edit the message header fields `From:',
           `Reply-To:' and `Sender:' by typing each one in turn and allowing
           the user to edit the field.  The default values for these fields
           originate from the from, reply-to and sender variables.  In non-
           interactive mode this sets ^ERR-NOTTY.
     ~h    In interactive mode, edit the message header fields `To:', `Cc:',
           `Bcc:' and `Subject:' by typing each one in turn and allowing the
           user to edit the field.  In non-interactive mode this sets
           ^ERR-NOTTY.
     ~I variable
           Insert the value of the specified variable into the message.  The
           message remains unaltered if the variable is unset or empty.  Any
           embedded character sequences `\t' horizontal tabulator and `\n'
           line feed are expanded in posix mode; otherwise the expansion
           should occur at set time ([v15 behaviour may differ] by using the
           command modifier wysh).
     ~i variable
           Like ~I, but appends a newline character.
     ~M messages
           Read the named messages into the message being sent, indented by
           indentprefix.  If no messages are specified, read the current mes-
           sage, the ``dot''.  Honours forward-add-cc as well as
           forward-inject-head and forward-inject-tail.
     ~m messages
           Read the named messages into the message being sent, indented by
           indentprefix.  If no messages are specified, read the current mes-
           sage, the ``dot''.  Strips down the list of header fields according
           to the `type' white- and blacklist selection of headerpick.  Hon-
           ours forward-add-cc as well as forward-inject-head and
           forward-inject-tail.  For MIME multipart messages, only the first
           displayable part is included.
     ~p    Display the message collected so far, prefaced by the message
           header fields and followed by the attachment list, if any.
     ~Q    Read in the given / current message(s) using the algorithm of quote
           (except that is implicitly assumed, even if not set), honouring
           quote-add-cc.
     ~q    Abort the message being sent, copying it to the file specified by
           the DEAD variable if save is set.
     ~R filename
           Identical to ~r, but indent each line that has been read by
           indentprefix.
     ~r filename [HERE-delimiter]
           Read the named file, object to Filename transformations excluding
           shell globs and variable expansions, into the message; if filename
           is the hyphen-minus `-' then standard input is used (for pasting,
           for example).  Only in this latter mode HERE-delimiter may be
           given: if it is data will be read in until the given HERE-delimiter
           is seen on a line by itself, and encountering EOF is an error; the
           HERE-delimiter is a required argument in non-interactive mode; if
           it is single-quote quoted then the pasted content will not be
           expanded, [v15 behaviour may differ] otherwise a future version of
           S-nail may perform shell-style expansion on the content.
     ~s string
           Cause the named string to become the current subject field.  New-
           line (NL) and carriage-return (CR) bytes are invalid and will be
           normalized to space (SP) characters.
     ~t name ...
           Add the given name(s) to the direct recipient list.
     ~U messages
           Read in the given / current message(s) excluding all headers,
           indented by indentprefix.  Honours forward-add-cc as well as
           forward-inject-head and forward-inject-tail.
     ~u messages
           Read in the given / current message(s), excluding all headers.
           Honours forward-add-cc as well as forward-inject-head and
           forward-inject-tail.
     ~v    Invoke the VISUAL editor on the message collected so far, then
           return to compose mode.  ~e can be used for a less display oriented
           editor, and ~|| offers a pipe-based editing approach.
     ~w filename
           Write the message onto the named file, which is object to the usual
           Filename transformations.  If the file exists, the message is
           appended to it.
     ~x    Same as ~q, except that the message is not saved at all.
INTERNAL VARIABLES
     Internal S-nail variables are controlled via the set and unset commands;
     prefixing a variable name with the string `no' and calling set has the
     same effect as using unset: `unset crt' and `set nocrt' do the same
     thing.  varshow will give more insight on the given variable(s), and set,
     when called without arguments, will show a listing of all variables.
     Both commands support a more verbose listing mode.  Some well-known vari-
     ables will also become inherited from the program ENVIRONMENT implicitly,
     others can be imported explicitly with the command environ and henceforth
     share said properties.
     Two different kinds of internal variables exist, and both of which can
     also form chains.  There are boolean variables, which can only be in one
     of the two states ``set'' and ``unset'', and value variables with a(n
     optional) string value.  For the latter proper quoting is necessary upon
     assignment time, the introduction of the section COMMANDS documents the
     supported quoting rules.
           ? wysh set one=val\ 1 two="val 2" \
               three='val "3"' four=$'val \'4\''; \
               varshow one two three four; \
               unset one two three four
     Dependent upon the actual option string values may become interpreted as
     colour names, command specifications, normal text, etc.  They may be
     treated as numbers, in which case decimal values are expected if so docu-
     mented, but otherwise any numeric format and base that is valid and
     understood by the vexpr command may be used, too.
     There also exists a special kind of string value, the ``boolean string'',
     which must either be a decimal integer (in which case `0' is false and
     `1' and any other value is true) or any of the (case-insensitive) strings
     `off', `no', `n' and `false' for a false boolean and `on', `yes', `y' and
     `true' for a true boolean; a special kind of boolean string is the
     ``quadoption'': it can optionally be prefixed with the (case-insensitive)
     term `ask-', as in `ask-yes'; in interactive mode the user will be
     prompted, otherwise the actual boolean is used.
     Variable chains extend a plain `variable' with `variable-HOST' and
     `variable-USER@HOST' variants.  Here `HOST' will be converted to all low-
     ercase when looked up (but not when the variable is set or unset!),
     [Option]ally IDNA converted, and indeed means `server:port' if a `port'
     had been specified in the contextual Uniform Resource Locator URL, see On
     URL syntax and credential lookup.  Even though this mechanism is based on
     URLs no URL percent encoding may be applied to neither of `USER' nor
     `HOST', variable chains need to be specified using raw data; the men-
     tioned section contains examples.  Variables which support chains are
     explicitly documented as such, and S-nail treats the base name of any
     such variable special, meaning that users should not create custom names
     like `variable-xyz' in order to avoid false classifications and treatment
     of such variables.
   Initial settings
     The standard POSIX 2008/Cor 2-2016 mandates the following initial vari-
     able settings: noallnet, noappend, asksub, noaskbcc, noautoprint, nobang,
     nocmd, nocrt, nodebug, nodot, escape set to `~', noflipr, nofolder,
     header, nohold, noignore, noignoreeof, nokeep, nokeepsave, nometoo,
     nooutfolder, nopage, prompt set to `? ', noquiet, norecord, save,
     nosendwait, noshowto, noSign, nosign, toplines set to `5'.
     However, S-nail has built-in some initial (and some default) settings
     which (may) diverge, others may become adjusted by one of the Resource
     files.  Displaying the former is accomplished via set: `$ s-nail -:/ -v
     -Xset -Xx'.  In general this implementation sets (and has extended the
     meaning of) sendwait, and does not support the noonehop variable - use
     command line options or mta-arguments to pass options through to a mta.
     The default global resource file sets, among others, the variables hold,
     keep and keepsave, establishes a default headerpick selection etc., and
     should thus be taken into account.
   Variables
     ?     (Read-only) The exit status of the last command, or the return
           value of the macro called last.  This status has a meaning in the
           state machine: in conjunction with errexit any non-0 exit status
           will cause a program exit, and in posix mode any error while load-
           ing (any of the) resource files will have the same effect.  ignerr,
           one of the Command modifiers, can be used to instruct the state
           machine to ignore errors.
     !     (Read-only) The current error number (errno(3)), which is set after
           an error occurred; it is also available via ^ERR, and the error
           name and documentation string can be queried via ^ERRNAME and
           ^ERRDOC.  [v15 behaviour may differ] This machinery is new and the
           error number is only really usable if a command explicitly states
           that it manages the variable !, for others errno will be used in
           case of errors, or ^ERR-INVAL if that is 0: it thus may or may not
           reflect the real error.  The error number may be set with the com-
           mand return.
     ^     (Read-only) This is a multiplexer variable which performs dynamic
           expansion of the requested state or condition, of which there are:
           ^ERR, ^ERRDOC, ^ERRNAME
                 The number, documentation, and name of the current errno(3),
                 respectively, which is usually set after an error occurred.
                 The documentation is an [Option], the name is used if not
                 available.  [v15 behaviour may differ] This machinery is new
                 and is usually reliable only if a command explicitly states
                 that it manages the variable !, which is effectively identi-
                 cal to ^ERR.  Each of those variables can be suffixed with a
                 hyphen minus followed by a name or number, in which case the
                 expansion refers to the given error.  Note this is a direct
                 mapping of (a subset of) the system error values:
                       define work {
                         eval echo \$1: \$^ERR-$1:\
                           \$^ERRNAME-$1: \$^ERRDOC-$1
                         vput vexpr i + "$1" 1
                         if [ $i -lt 16 ]
                           \xcall work $i
                         end
                       }
                       call work 0
           ^ERRQUEUE-COUNT, ^ERRQUEUE-EXISTS
                 The number of messages in the [Option]al queue of errors, and
                 a string indicating queue state: empty or (translated)
                 ``ERROR''.  Always 0 and the empty string, respectively,
                 unless features includes `,+errors,'.
     *     (Read-only) Expands all positional parameters (see 1), separated by
           the first character of the value of ifs.  [v15 behaviour may dif-
           fer] The special semantics of the equally named special parameter
           of the sh(1) are not yet supported.
     @     (Read-only) Expands all positional parameters (see 1), separated by
           a space character.  If placed in double quotation marks, each posi-
           tional parameter is properly quoted to expand to a single parameter
           again.
     #     (Read-only) Expands to the number of positional parameters, i.e.,
           the size of the positional parameter stack in decimal.
     0     (Read-only) Inside the scope of a defined and called macro this
           expands to the name of the calling macro, or to the empty string if
           the macro is running from top-level.  For the [Option]al regular
           expression search and replace operator of vexpr this expands to the
           entire matching expression.  It represents the program name in
           global context.
     1     (Read-only) Access of the positional parameter stack.  All further
           parameters can be accessed with this syntax, too, `2', `3' etc.;
           positional parameters can be shifted off the stack by calling
           shift.  The parameter stack contains, for example, the arguments of
           a called defined macro, the matching groups of the [Option]al regu-
           lar expression search and replace expression of vexpr, and can be
           explicitly created or overwritten with the command vpospar.
     account
           (Read-only) Is set to the active account.
     add-file-recipients
           (Boolean) When file or pipe recipients have been specified, mention
           them in the corresponding address fields of the message instead of
           silently stripping them from their recipient list.  By default such
           addressees are not mentioned.
     allnet
           (Boolean) Causes only the local part to be evaluated when comparing
           addresses.
     append
           (Boolean) Causes messages saved in the secondary mailbox MBOX to be
           appended to the end rather than prepended.  This should always be
           set.
     askatend
           (Boolean) Causes the prompts for `Cc:' and `Bcc:' lists to appear
           after the message has been edited.
     askattach
           (Boolean) If set, S-nail asks an interactive user for files to
           attach at the end of each message; An empty line finalizes the
           list.
     askcc
           (Boolean) Causes the interactive user to be prompted for carbon
           copy recipients (at the end of each message if askatend or
           bsdcompat are set).
     askbcc
           (Boolean) Causes the interactive user to be prompted for blind car-
           bon copy recipients (at the end of each message if askatend or
           bsdcompat are set).
     asksend
           (Boolean) Causes the interactive user to be prompted for confirma-
           tion to send the message or reenter compose mode after having been
           shown a preliminary envelope summary.
     asksign
           (Boolean)[Option] Causes the interactive user to be prompted if the
           message is to be signed at the end of each message.  The smime-sign
           variable is ignored when this variable is set.
     asksub
           (Boolean) Causes S-nail to prompt the interactive user for the sub-
           ject upon entering compose mode unless a subject already exists.
     attrlist
           A sequence of characters to display in the `attribute' column of
           the headline as shown in the display of headers; each for one type
           of messages (see Message states), with the default being
           `NUROSPMFAT+-$~' or `NU  *HMFAT+-$~' if the bsdflags variable is
           set, in the following order:
           `N'  new.
           `U'  unread but old.
           `R'  new but read.
           `O'  read and old.
           `S'  saved.
           `P'  preserved.
           `M'  mboxed.
           `F'  flagged.
           `A'  answered.
           `T'  draft.
           `+'  [v15 behaviour may differ] start of a (collapsed) thread in
                threaded mode (see autosort, thread);
           `-'  [v15 behaviour may differ] an uncollapsed thread in threaded
                mode; only used in conjunction with -L.
           `$'  classified as spam.
           `~'  classified as possible spam.
     autobcc
           Specifies a list of recipients to which a blind carbon copy of each
           outgoing message will be sent automatically.
     autocc
           Specifies a list of recipients to which a carbon copy of each out-
           going message will be sent automatically.
     autocollapse
           (Boolean) Causes threads to be collapsed automatically when .Ql
           thread Ns ed sort mode is entered (see the collapse command).
     autoprint
           (Boolean) Enable automatic typeing of a(n existing) ``successive''
           message after delete and undelete commands: the message that
           becomes the new ``dot'' is shown automatically, as via dp or dt.
     autosort
           Causes sorted mode (see the sort command) to be entered automati-
           cally with the value of this variable as sorting method when a
           folder is opened, for example `set autosort=thread'.
     bang  (Boolean) Enables the substitution of all not (reverse-solidus)
           escaped exclamation mark `!' characters by the contents of the last
           executed command for the ! shell escape command and ~!, one of the
           compose mode COMMAND ESCAPES.  If this variable is not set no
           reverse solidus stripping is performed.
     bind-timeout
           [Obsolete] Predecessor of bind-inter-byte-timeout.  [v15 behaviour
           may differ] Setting this automatically sets the successor.
     bind-inter-byte-timeout
           [Option] Terminals may generate multi-byte sequences for special
           function keys, for example, but these sequences may not become read
           as a unit.  And multi-byte sequences can be defined freely via
           bind.  This variable specifies the timeout in milliseconds that the
           MLE (see On terminal control and line editor) waits for more bytes
           to arrive unless it considers a sequence ``complete''.  The default
           is 200, the maximum is about 10 seconds.  In the following example
           the comments state which sequences are affected by this timeout:
                 ? bind base abc echo 0 # abc
                 ? bind base ab,c echo 1 # ab
                 ? bind base abc,d echo 2 # abc
                 ? bind base ac,d echo 3 # ac
                 ? bind base a,b,c echo 4
                 ? bind base a,b,c,d echo 5
                 ? bind base a,b,cc,dd echo 6 # cc and dd
     bind-inter-key-timeout
           [Option] Multi-key bind sequences do not time out by default.  If
           this variable is set, then the current key sequence is forcefully
           terminated once the timeout (in milliseconds) triggers.  The value
           should be (maybe significantly) larger than
           bind-inter-byte-timeout, but may not excess the maximum, too.
     bsdcompat
           (Boolean) Sets some cosmetical features to traditional BSD style;
           has the same affect as setting askatend and all other variables
           prefixed with `bsd'; it also changes the behaviour of emptystart
           (which does not exist in BSD).
     bsdflags
           (Boolean) Changes the letters shown in the first column of a header
           summary to traditional BSD style.
     bsdheadline
           (Boolean) Changes the display of columns in a header summary to
           traditional BSD style.
     bsdmsgs
           (Boolean) Changes some informational messages to traditional BSD
           style.
     bsdorder
           (Boolean) Causes the `Subject:' field to appear immediately after
           the `To:' field in message headers and with the ~h COMMAND ESCAPES.
     build-cc, build-ld, build-os, build-rest
           (Read-only) The build environment, including the compiler, the
           linker, the operating system S-nail has been build for, usually
           taken from uname(1) via `uname -s', and then lowercased, as well as
           all the possibly interesting rest of the configuration and build
           environment.  This information is also available in the verbose
           output of the command version.
     charset-7bit
           The value that should appear in the `charset=' parameter of
           `Content-Type:' MIME header fields when no character set conversion
           of the message data was performed.  This defaults to US-ASCII, and
           the chosen character set should be US-ASCII compatible.
     charset-8bit
           [Option] The default 8-bit character set that is used as an
           implicit last member of the variable sendcharsets.  This defaults
           to UTF-8 if character set conversion capabilities are available,
           and to ISO-8859-1 otherwise (unless the operating system environ-
           ment is known to always and exclusively support UTF-8 locales), in
           which case the only supported character set is ttycharset and this
           variable is effectively ignored.
     charset-unknown-8bit
           [Option] RFC 1428 specifies conditions when internet mail gateways
           shall ``upgrade'' the content of a mail message by using a charac-
           ter set with the name `unknown-8bit'.  Because of the unclassified
           nature of this character set S-nail will not be capable to convert
           this character set to any other character set.  If this variable is
           set any message part which uses the character set `unknown-8bit' is
           assumed to really be in the character set given in the value, oth-
           erwise the (final) value of charset-8bit is used for this purpose.
           This variable will also be taken into account if a MIME type (see
           The mime.types files) of a MIME message part that uses the `binary'
           character set is forcefully treated as text.
     cmd   The default value for the pipe command.
     colour-disable
           (Boolean)[Option] Forcefully disable usage of colours.  Also see
           the section Coloured display.
     colour-pager
           (Boolean)[Option] Whether colour shall be used for output that is
           paged through PAGER.  Note that pagers may need special command
           line options, for example less(1) requires the option -R and lv(1)
           the option -c in order to support colours.  Often doing manual
           adjustments is unnecessary since S-nail may perform adjustments
           dependent on the value of the environment variable PAGER (see there
           for more).
     contact-mail, contact-web
           (Read-only) Addresses for contact per email and web, respectively,
           for bug reports, suggestions, or anything else regarding S-nail.
           The former can be used directly: `? eval mail $contact-mail'.
     content-description-forwarded-message,
           content-description-quote-attachment,
           content-description-smime-message,
           content-description-smime-signature
           [Option](partially) Strings which will be placed in according
           `Content-Description:' headers if non-empty.  They all have default
           values, for example `Forwarded message'.
     crt   In a(n interactive) terminal session, then if this valued variable
           is set it will be used as a threshold to determine how many lines
           the given output has to span before it will be displayed via the
           configured PAGER; Usage of the PAGER can be forced by setting this
           to the value `0', setting it without a value will deduce the cur-
           rent height of the terminal screen to compute the threshold (see
           LINES, screen and stty(1)).  [v15 behaviour may differ] At the
           moment this uses the count of lines of the message in wire format,
           which, dependent on the mime-encoding of the message, is unrelated
           to the number of display lines.  (The software is old and histori-
           cally the relation was a given thing.)
     customhdr
           Define a set of custom headers to be injected into newly composed
           or forwarded messages.  A custom header consists of the field name
           followed by a colon `:' and the field content body.  Standard
           header field names cannot be overwritten by a custom header, with
           the exception of `Comments:' and `Keywords:'.  Different to the
           command line option -C the variable value is interpreted as a
           comma-separated list of custom headers: to include commas in header
           bodies they need to become escaped with reverse solidus `\'.  Head-
           ers can be managed more freely in Compose mode via ~^.
                 ? set customhdr='Hdr1: Body1-1\, Body1-2, Hdr2: Body2'
     datefield
           Controls the appearance of the `%d' date and time format specifica-
           tion of the headline variable, that is used, for example, when
           viewing the summary of headers.  If unset, then the local receiving
           date is used and displayed unformatted, otherwise the message send-
           ing `Date:'.  It is possible to assign a strftime(3) format string
           and control formatting, but embedding newlines via the `%n' format
           is not supported, and will result in display errors.  The default
           is `%Y-%m-%d %H:%M', and also see datefield-markout-older.
     datefield-markout-older
           Only used in conjunction with datefield.  Can be used to create a
           visible distinction of messages dated more than a day in the
           future, or older than six months, a concept comparable to the -l
           option of the POSIX utility ls(1).  If set to the empty string,
           then the plain month, day and year of the `Date:' will be dis-
           played, but a strftime(3) format string to control formatting can
           be assigned.  The default is `%Y-%m-%d'.
     debug
           (Boolean) (Almost) Enter a debug-only sandbox mode which generates
           many log messages, disables the actual delivery of messages, and
           also implies norecord as well as nosave.  Also see verbose.
     disposition-notification-send
           (Boolean)[Option] Emit a `Disposition-Notification-To:' header (RFC
           3798) with the message.  This requires the from variable to be set.
     dot   (Boolean) When dot is set, a period `.' on a line by itself during
           message input in (interactive or batch -#) Compose mode will be
           treated as end-of-message (in addition to the normal end-of-file
           condition).  This behaviour is implied in posix mode with a set
           ignoreeof.
     dotlock-disable
           (Boolean)[Option] Disable creation of dotlock files for MBOX data-
           bases.
     dotlock-ignore-error
           [Obsolete](Boolean)[Option] Ignore failures when creating dotlock
           files.  Please use dotlock-disable instead.
     editalong
           If this variable is set then the editor is started automatically
           when a message is composed in interactive mode.  If the value
           starts with the letter `v' then this acts as if ~v, otherwise as if
           ~e (see COMMAND ESCAPES) had been specified.  The editheaders vari-
           able is implied for this automatically spawned editor session.
     editheaders
           (Boolean) When a message is edited while being composed, its header
           is included in the editable text.
     emptystart
           (Boolean) When entering interactive mode S-nail normally writes
           ``No mail for user'' and exits immediately if a mailbox is empty or
           does not exist.  If this variable is set S-nail starts even with an
           empty or non-existent mailbox (the latter behaviour furtherly
           depends upon bsdcompat, though).
     errexit
           (Boolean) Let each command with a non-0 exit status, including
           every called macro which returns a non-0 status, cause a program
           exit unless prefixed by ignerr (see Command modifiers).  This also
           affects COMMAND ESCAPES, but which use a different modifier for
           ignoring the error.  Please refer to the variable ? for more on
           this topic.
     errors-limit
           [Option] Maximum number of entries in the errors queue.
     escape
           The first character of this value defines the escape character for
           COMMAND ESCAPES in Compose mode.  The default value is the charac-
           ter tilde `~'.  If set to the empty string, command escapes are
           disabled.
     expandaddr
           If unset only user name and email address recipients are allowed On
           sending mail, and non-interactive mode.  If set without value all
           possible recipient types will be accepted.  A value is parsed as a
           comma-separated list of case-insensitive strings, and if that con-
           tains `restrict' behaviour equals the former except when in inter-
           active mode or if COMMAND ESCAPES were enabled via -~ or -#, in
           which case it equals the latter, allowing all address types.
           `restrict' really acts like `restrict,-all,+name,+addr', so care
           for ordering issues must be taken.
           Recipient types can be added and removed with a plus sign `+' or
           hyphen-minus `-' prefix, respectively.  By default invalid or dis-
           allowed types are filtered out and cause a warning, hard send
           errors need to be enforced by including `fail'.  The value `all'
           covers all types, `fcc' whitelists `Fcc:' header targets regardless
           of other settings, `file' file targets (it includes `fcc'), `pipe'
           command pipeline targets, `name' user names still unexpanded after
           alias and mta-aliases processing and thus left for expansion by the
           mta (invalid for the built-in SMTP one), and `addr' network
           addresses.  Targets are interpreted in the given order, so that
           `restrict,fail,+file,-all,+addr' will cause hard errors for any
           non-network address recipient address unless running interactively
           or having been started with the option -~ or -#; in the latter
           case(s) any type may be used.
           User name receivers addressing valid local users can be expanded to
           fully qualified network addresses (also see hostname) by including
           `nametoaddr' in the list.  Historically invalid recipients were
           stripped off without causing errors, this can be changed by making
           `failinvaddr' an entry of the list (it really acts like
           `failinvaddr,+addr').  Likewise, `domaincheck' (really
           `domaincheck,+addr') compares address domain names against a
           whitelist and strips off (`fail' for hard errors) addressees which
           fail this test; the domain name `localhost' and the non-empty value
           of hostname (the real hostname otherwise) are always whitelisted,
           expandaddr-domaincheck can be set to extend this list.  Finally
           some address providers (for example -b, -c and all other command
           line recipients) will be evaluated as if specified within dollar-
           single-quotes (see Shell-style argument quoting) if the value list
           contains the string `shquote'.
     expandaddr-domaincheck
           Can be set to a comma-separated list of domain names which should
           be whitelisted for the evaluation of the `domaincheck' mode of
           expandaddr.  IDNA encoding is not automatically performed,
           addrcodec can be used to prepare the domain (of an address).
     expandargv
           Unless this variable is set additional mta (Mail-Transfer-Agent)
           arguments from the command line, as can be given after a -- separa-
           tor, results in a program termination with failure status.  The
           same can be accomplished by using the special (case-insensitive)
           value `fail'.  A lesser strict variant is the otherwise identical
           `restrict', which does accept such arguments in interactive mode,
           or if tilde commands were enabled explicitly by using one of the
           command line options -~ or -#.  The empty value will allow uncondi-
           tional usage.
     features
           (Read-only) String giving a list of optional features.  Features
           are preceded with a plus sign `+' if they are available, with a
           hyphen-minus `-' otherwise.  To ease substring matching the string
           starts and ends with a comma.  The output of the command version
           includes this information in a more pleasant output.
     flipr
           (Boolean) This setting reverses the meanings of a set of reply com-
           mands, turning the lowercase variants, which by default address all
           recipients included in the header of a message (reply, respond,
           followup) into the uppercase variants, which by default address the
           sender only (Reply, Respond, Followup) and vice versa.
     folder
           The default path under which mailboxes are to be saved: filenames
           that begin with the plus sign `+' will have the plus sign replaced
           with the value of this variable if set, otherwise the plus sign
           will remain unchanged when doing Filename transformations; also see
           folder for more on this topic, and know about standard imposed
           implications of outfolder.  The value supports a subset of trans-
           formations itself, and if the non-empty value does not start with a
           solidus `/', then the value of HOME will be prefixed automatically.
           Once the actual value is evaluated first, the internal variable
           folder-resolved will be updated for caching purposes.
     folder-hook-FOLDER, folder-hook
           Names a defined macro which will be called whenever a folder is
           opened.  The macro will also be invoked when new mail arrives, but
           message lists for commands executed from the macro only include
           newly arrived messages then.  localopts are activated by default in
           a folder hook, causing the covered settings to be reverted once the
           folder is left again.
           The specialized form will override the generic one if `FOLDER'
           matches the file that is opened.  Unlike other folder specifica-
           tions, the fully expanded name of a folder, without metacharacters,
           is used to avoid ambiguities.  However, if the mailbox resides
           under folder then the usual `+' specification is tried in addition,
           so that if folder is ``mail'' (and thus relative to the user's home
           directory) then /home/usr1/mail/sent will be tried as
           `folder-hook-/home/usr1/mail/sent' first, but then followed by
           `folder-hook-+sent'.
     folder-resolved
           (Read-only) Set to the fully resolved path of folder once that
           evaluation has occurred; rather internal.
     followup-to
           (Boolean) Controls whether a `Mail-Followup-To:' header is gener-
           ated when sending messages to known mailing lists.  The user as
           determined via from (or, if that contains multiple addresses,
           sender) will be placed in there if any list addressee is not a sub-
           scribed list.  Also see followup-to-honour and the commands mlist,
           mlsubscribe, reply and Lreply.
     followup-to-add-cc
           (Boolean) Controls whether the user will be added to the messages'
           `Cc:' list in addition to placing an entry in `Mail-Followup-To:'
           (see followup-to).
     followup-to-honour
           Controls whether a `Mail-Followup-To:' header is honoured when
           group-replying to a message via reply or Lreply.  This is a
           quadoption; if set without a value it defaults to ``yes'', and see
           followup-to.
     forward-add-cc
           (Boolean) Whether senders of messages forwarded via ~F, ~f, ~m, ~U
           or ~u shall be made members of the carbon copies `Cc:' list.
     forward-as-attachment
           (Boolean) Original messages are normally sent as inline text with
           the forward command, and only the first part of a multipart message
           is included.  With this setting enabled messages are sent as unmod-
           ified MIME `message/rfc822' attachments with all of their parts
           included.
     forward-inject-head, forward-inject-tail
           The strings to put before and after the text of a message with the
           forward command, respectively.  The former defaults to `--------
           Original Message --------\n'.  Special format directives in these
           strings will be expanded if possible, and if so configured the out-
           put will be folded according to quote-fold; for more please refer
           to quote-inject-head.  Injections will not be performed by forward
           if the variable forward-as-attachment is set -- the COMMAND ESCAPES
           ~F, ~f, ~M, ~m, ~U, ~u always inject.
     from  The address (or a list of addresses) to put into the `From:' field
           of the message header, quoting RFC 5322: the author(s) of the mes-
           sage, that is, the mailbox(es) of the person(s) or system(s)
           responsible for the writing of the message.  According to that RFC
           setting the sender variable is required if from contains more than
           one address.  [v15 behaviour may differ] Please expect automatic
           management of the from and sender relationship.  Dependent on the
           context these addresses are handled as if they were in the list of
           alternates.
           If a file-based MTA is used, then from (or, if that contains multi-
           ple addresses, sender) can nonetheless be used as the envelope
           sender address at the MTA protocol level (the RFC 5321 reverse-
           path), either via the -r command line option (without argument; see
           there for more), or by setting r-option-implicit.
           If the machine's hostname is not valid at the Internet (for example
           at a dialup machine), then either this variable or hostname
           ([v15-compat] a SMTP-based mta adds even more fine-tuning capabili-
           ties with smtp-hostname) have to be set: if so the message and MIME
           part related unique ID fields `Message-ID:' and `Content-ID:' will
           be created (except when disallowed by message-id-disable or
           stealthmua).
     fullnames
           (Boolean) Due to historical reasons comments and name parts of
           email addresses are removed by default when sending mail, replying
           to or forwarding a message.  If this variable is set such stripping
           is not performed.
     fwdheading
           [Obsolete] Predecessor of forward-inject-head.
     header
           (Boolean) Causes the header summary to be written at startup and
           after commands that affect the number of messages or the order of
           messages in the current folder.  Unless in posix mode a header sum-
           mary will also be displayed on folder changes.  The command line
           option -N can be used to set noheader.
     headline
           A format string to use for the summary of headers.  Format speci-
           fiers in the given string start with a percent sign `%' and may be
           followed by an optional decimal number indicating the field width
           -- if that is negative, the field is to be left-aligned.  Names and
           addresses are subject to modifications according to showname and
           showto.  Valid format specifiers are:
           `%%'    A plain percent sign.
           `%>'    ``Dotmark'': a space character but for the current message
                   (``dot''), for which it expands to `>' (dependent on
                   headline-plain).
           `%<'    ``Dotmark'': a space character but for the current message
                   (``dot''), for which it expands to `<' (dependent on
                   headline-plain).
           `%$'    [Option] The spam score of the message, as has been classi-
                   fied via the command spamrate.  Shows only a replacement
                   character if there is no spam support.
           `%a'    Message attribute character (status flag); the actual con-
                   tent can be adjusted by setting attrlist.
           `%d'    The date found in the `Date:' header of the message when
                   datefield is set (the default), otherwise the date when the
                   message was received.  Formatting can be controlled by
                   assigning a strftime(3) format string to datefield (and
                   datefield-markout-older).
           `%e'    The indenting level in `thread'ed sort mode.
           `%f'    The address of the message sender.
           `%i'    The message thread tree structure.  (Note that this format
                   does not support a field width, and honours
                   headline-plain.)
           `%L'    Mailing list status: is the addressee of the message a
                   known `l' (mlist) or `L' mlsubscribed mailing list?  The
                   letter `P' announces the presence of a RFC 2369
                   `List-Post:' header, which makes a message a valuable tar-
                   get of Lreply.
           `%l'    The number of lines of the message, if available.
           `%m'    Message number.
           `%o'    The number of octets (bytes) in the message, if available.
           `%S'    Message subject (if any) in double quotes.
           `%s'    Message subject (if any).
           `%t'    The position in threaded/sorted order.
           `%U'    The value 0 except in an IMAP mailbox, where it expands to
                   the UID of the message.
           The default is `%>%a%m %-18f %16d %4l/%-5o %i%-s', or
           `%>%a%m %20-f  %16d %3l/%-5o %i%-S' if bsdcompat is set.  Also see
           attrlist, headline-plain and headline-bidi.
     headline-bidi
           Bidirectional text requires special treatment when displaying head-
           ers, because numbers (in dates or for file sizes etc.) will not
           affect the current text direction, in effect resulting in ugly line
           layouts when arabic or other right-to-left text is to be displayed.
           On the other hand only a minority of terminals is capable to cor-
           rectly handle direction changes, so that user interaction is neces-
           sary for acceptable results.  Note that extended host system sup-
           port is required nonetheless, e.g., detection of the terminal char-
           acter set is one precondition; and this feature only works in an
           Unicode (i.e., UTF-8) locale.
           In general setting this variable will cause S-nail to encapsulate
           text fields that may occur when displaying headline (and some other
           fields, like dynamic expansions in prompt) with special Unicode
           control sequences; it is possible to fine-tune the terminal support
           level by assigning a value: no value (or any value other than `1',
           `2' and `3') will make S-nail assume that the terminal is capable
           to properly deal with Unicode version 6.3, in which case text is
           embedded in a pair of U+2068 (FIRST STRONG ISOLATE) and U+2069 (POP
           DIRECTIONAL ISOLATE) characters.  In addition no space on the line
           is reserved for these characters.
           Weaker support is chosen by using the value `1' (Unicode 6.3, but
           reserve the room of two spaces for writing the control sequences
           onto the line).  The values `2' and `3' select Unicode 1.1 support
           (U+200E, LEFT-TO-RIGHT MARK); the latter again reserves room for
           two spaces in addition.
     headline-plain
           (Boolean) On Unicode (UTF-8) aware terminals enhanced graphical
           symbols are used by default for certain entries of headline.  If
           this variable is set only basic US-ASCII symbols will be used.
     history-file
           [Option] The (expandable) location of a permanent history file for
           the MLE line editor (On terminal control and line editor).  Also
           see history-size.
     history-gabby
           [Option] Add more entries to the MLE history as is normally done.
           A comma-separated list of case-insensitive strings can be used to
           fine-tune which gabby entries shall be allowed.  If it contains
           `errors', erroneous commands will also be added.  `all' adds all
           optional entries, and is the fallback chattiness identifier of
           on-history-addition.
     history-gabby-persist
           (Boolean)[Option] The history-gabby entries will not be saved in
           persistent storage unless this variable is set.  The knowledge of
           whether a persistent entry was gabby is not lost.  Also see
           history-file.
     history-size
           [Option] Setting this variable imposes a limit on the number of
           concurrent history entries.  If set to the value 0 then no further
           history entries will be added, and loading and incorporation of the
           history-file upon program startup can also be suppressed by doing
           this.  Runtime changes will not be reflected before the history is
           saved or loaded (again).
     hold  (Boolean) This setting controls whether messages are held in the
           system inbox, and it is set by default.
     hostname
           Used instead of the value obtained from uname(3) and getaddrinfo(3)
           as the hostname when expanding local addresses, for example in
           `From:' (also see On sending mail, and non-interactive mode, for
           expansion of addresses that have a valid user-, but no domain name
           in angle brackets).  If either of from or this variable is set the
           message and MIME part related unique ID fields `Message-ID:' and
           `Content-ID:' will be created (except when disallowed by
           message-id-disable or stealthmua).  If the [Option]al IDNA support
           is available (see idna-disable) variable assignment is aborted when
           a necessary conversion fails.
           Setting it to the empty string will cause the normal hostname to be
           used, but nonetheless enables creation of said ID fields.
           [v15-compat] in conjunction with the built-in SMTP mta
           smtp-hostname also influences the results: one should produce some
           test messages with the desired combination of hostname, and/or
           from, sender etc. first.
     idna-disable
           (Boolean)[Option] Can be used to turn off the automatic conversion
           of domain names according to the rules of IDNA (internationalized
           domain names for applications).  Since the IDNA code assumes that
           domain names are specified with the ttycharset character set, an
           UTF-8 locale charset is required to represent all possible interna-
           tional domain names (before conversion, that is).
     ifs   The input field separator that is used ([v15 behaviour may differ]
           by some functions) to determine where to split input data.
           1.   Unsetting is treated as assigning the default value, ` \t\n'.
           2.   If set to the empty value, no field splitting will be per-
                formed.
           3.   If set to a non-empty value, all whitespace characters are
                extracted and assigned to the variable ifs-ws.
           a.   ifs-ws will be ignored at the beginning and end of input.
                Diverging from POSIX shells default whitespace is removed in
                addition, which is owed to the entirely different line content
                extraction rules.
           b.   Each occurrence of a character of ifs will cause field-split-
                ting, any adjacent ifs-ws characters will be skipped.
     ifs-ws
           (Read-only) Automatically deduced from the whitespace characters in
           ifs.
     ignore
           (Boolean) Ignore interrupt signals from the terminal while entering
           messages; instead echo them as `@' characters and discard the cur-
           rent line.
     ignoreeof
           (Boolean) Ignore end-of-file conditions (`control-D') in Compose
           mode on message input and in interactive command input.  If set an
           interactive command input session can only be left by explicitly
           using one of the commands exit and quit, and message input in com-
           pose mode can only be terminated by entering a period `.' on a line
           by itself or by using the ~. COMMAND ESCAPES; Setting this implies
           the behaviour that dot describes in posix mode.
     inbox
           If this is set to a non-empty string it will specify the user's
           primary system mailbox, overriding MAIL and the system-dependent
           default, and (thus) be used to replace `%' when doing Filename
           transformations; also see folder for more on this topic.  The value
           supports a subset of transformations itself.
     indentprefix
           String used by the ~m, ~M and ~R COMMAND ESCAPES and by the quote
           option for indenting messages, in place of the POSIX mandated
           default tabulator character `\t'.  Also see quote-chars.
     keep  (Boolean) If set, an empty primary system mailbox file is not
           removed.  Note that, in conjunction with posix mode any empty file
           will be removed unless this variable is set.  This may improve the
           interoperability with other mail user agents when using a common
           folder directory, and prevents malicious users from creating fake
           mailboxes in a world-writable spool directory.  [v15 behaviour may
           differ] Only local regular (MBOX) files are covered, Maildir and
           other mailbox types will never be removed, even if empty.
     keep-content-length
           (Boolean) When (editing messages and) writing MBOX mailbox files
           S-nail can be told to keep the `Content-Length:' and `Lines:'
           header fields that some MUAs generate by setting this variable.
           Since S-nail does neither use nor update these non-standardized
           header fields (which in itself shows one of their conceptual prob-
           lems), stripping them should increase interoperability in between
           MUAs that work with with same mailbox files.  Note that, if this is
           not set but writebackedited, as below, is, a possibly performed
           automatic stripping of these header fields already marks the mes-
           sage as being modified.  [v15 behaviour may differ] At some future
           time S-nail will be capable to rewrite and apply an mime-encoding
           to modified messages, and then those fields will be stripped
           silently.
     keepsave
           (Boolean) When a message is saved it is usually discarded from the
           originating folder when S-nail is quit.  This setting causes all
           saved message to be retained.
     line-editor-cpl-word-breaks
           [Option] List of bytes which are used by the mle-complete tabulator
           completion to decide where word boundaries exist, by default
           `"'@=;|:' [v15 behaviour may differ] This mechanism is yet
           restricted.
     line-editor-disable
           (Boolean) Turn off any line editing capabilities (from S-nails POW,
           see On terminal control and line editor for more).
     line-editor-no-defaults
           (Boolean)[Option] Do not establish any default key binding.
     log-prefix
           Error log message prefix string (`s-nail: ').
     mailbox-display
           (Read-only) The name of the current mailbox (folder), possibly
           abbreviated for display purposes.
     mailbox-resolved
           (Read-only) The fully resolved path of the current mailbox.
     mailcap-disable
           (Boolean)[Option] Turn off consideration of MIME type handlers
           from, and implicit loading of The Mailcap files.
     mailx-extra-rc
           An additional startup file that is loaded as the last of the
           Resource files.  Use this file for commands that are not understood
           by other POSIX mailx(1) implementations, i.e., mostly anything
           which is not covered by Initial settings.
     markanswered
           (Boolean) When a message is replied to and this variable is set, it
           is marked as having been answered.  See the section Message states.
     mbox-fcc-and-pcc
           (Boolean) By default all file and pipe message receivers (see
           expandaddr) will be fed valid MBOX database entry message data (see
           folder, mbox-rfc4155), and existing file targets will become
           extended in compliance to RFC 4155.  If this variable is unset then
           a plain standalone RFC 5322 message will be written, and existing
           file targets will be overwritten.
     mbox-rfc4155
           (Boolean) When opening MBOX mailbox databases, and in order to
           achieve compatibility with old software, the very tolerant POSIX
           standard rules for detecting message boundaries (so-called `From_'
           lines) are used instead of the stricter rules from the standard RFC
           4155.  This behaviour can be switched by setting this variable.
           This may temporarily be handy when S-nail complains about invalid
           `From_' lines when opening a MBOX: in this case setting this vari-
           able and re-opening the mailbox in question may correct the result.
           If so, copying the entire mailbox to some other file, as in `copy *
           SOME-FILE', will perform proper, all-compatible `From_' quoting for
           all detected messages, resulting in a valid MBOX mailbox.  ([v15
           behaviour may differ] The better and non-destructive approach is to
           re-encode invalid messages, as if it would be created anew, instead
           of mangling the `From_' lines; this requires the structural code
           changes of the v15 rewrite.)  Finally the variable can be unset
           again:
                 define mboxfix {
                   localopts yes; wysh set mbox-rfc4155;\
                     wysh File "${1}"; copy * "${2}"
                 }
                 call mboxfix /tmp/bad.mbox /tmp/good.mbox
     memdebug
           (Boolean) Internal development variable.  (Keeps memory debug
           enabled even if debug is not set.)
     message-id-disable
           (Boolean) By setting this variable the generation of `Message-ID:'
           and `Content-ID:' message and MIME part headers can be completely
           suppressed, effectively leaving this task up to the mta (Mail-
           Transfer-Agent) or the SMTP server.  Note that according to RFC
           5321 a SMTP server is not required to add this field by itself, so
           it should be ensured that it accepts messages without `Message-ID'.
     message-inject-head
           A string to put at the beginning of each new message, followed by a
           newline.  [Obsolete] The escape sequences tabulator `\t' and new-
           line `\n' are understood (use the wysh prefix when setting the
           variable(s) instead).
     message-inject-tail
           A string to put at the end of each new message, followed by a new-
           line.  [Obsolete] The escape sequences tabulator `\t' and newline
           `\n' are understood (use the wysh prefix when setting the vari-
           able(s) instead).  Also see on-compose-leave.
     metoo
           (Boolean) Usually, when an alias expansion contains the sender, the
           sender is removed from the expansion.  Setting this option sup-
           presses these removals.  Note that a set metoo also causes a `-m'
           option to be passed through to the mta (Mail-Transfer-Agent);
           though most of the modern MTAs no longer document this flag, no MTA
           is known which does not support it (for historical compatibility).
     mime-allow-text-controls
           (Boolean) When sending messages, each part of the message is MIME-
           inspected in order to classify the `Content-Type:' and
           `Content-Transfer-Encoding:' (see mime-encoding) that is required
           to send this part over mail transport, i.e., a computation rather
           similar to what the file(1) command produces when used with the
           `--mime' option.
           This classification however treats text files which are encoded in
           UTF-16 (seen for HTML files) and similar character sets as binary
           octet-streams, forcefully changing any `text/plain' or `text/html'
           specification to `application/octet-stream': If that actually hap-
           pens a yet unset charset MIME parameter is set to `binary', effec-
           tively making it impossible for the receiving MUA to automatically
           interpret the contents of the part.
           If this variable is set, and the data was unambiguously identified
           as text data at first glance (by a `.txt' or `.html' file exten-
           sion), then the original `Content-Type:' will not be overwritten.
     mime-alternative-favour-rich
           (Boolean) If this variable is set then rich MIME alternative parts
           (e.g., HTML) will be preferred in favour of included plain text
           versions when displaying messages, provided that a handler exists
           which produces output that can be (re)integrated into S-nail's nor-
           mal visual display.
     mime-counter-evidence
           Normally the `Content-Type:' field is used to decide how to handle
           MIME parts.  Some MUAs, however, do not use The mime.types files
           (also see HTML mail and MIME attachments) or a similar mechanism to
           correctly classify content, but specify an unspecific MIME type
           (`application/octet-stream') even for plain text attachments.  If
           this variable is set then S-nail will try to re-classify such MIME
           message parts, if possible, for example via a possibly existing
           attachment filename.  A non-empty value may also be given, in which
           case a number is expected, actually a carrier of bits, best speci-
           fied as a binary value, like `0b1111'.
           o   If bit two is set (counting from 1, decimal 2) then the
               detected mimetype will be carried along with the message and be
               used for deciding which MIME handler is to be used, for exam-
               ple; when displaying such a MIME part the part-info will indi-
               cate the overridden content-type by showing a plus sign `+'.
           o   If bit three is set (decimal 4) then the counter-evidence is
               always produced and a positive result will be used as the MIME
               type, even forcefully overriding the parts given MIME type.
           o   If bit four is set (decimal 8) as a last resort the actual con-
               tent of `application/octet-stream' parts will be inspected, so
               that data which looks like plain text can be treated as such.
               This mode is even more relaxed when data is to be displayed to
               the user or used as a message quote (data consumers which man-
               gle data for display purposes, which includes masking of con-
               trol characters, for example).
     mime-encoding
           The MIME `Content-Transfer-Encoding' to use in outgoing text mes-
           sages and message parts, where applicable (7-bit clean text mes-
           sages are without an encoding if possible):
           `8bit'  (Or `8b'.)  8-bit transport effectively causes the raw data
                   be passed through unchanged, but may cause problems when
                   transferring mail messages over channels that are not ESMTP
                   (RFC 1869) compliant.  Also, several input data constructs
                   are not allowed by the specifications and may cause a dif-
                   ferent transfer-encoding to be used.  By established rules
                   and popular demand occurrences of `^From_' (see
                   mbox-rfc4155) will be MBOXO quoted (prefixed with greater-
                   than sign `>') instead of causing a non-destructive encod-
                   ing like `quoted-printable' to be chosen, unless context
                   (like message signing) requires otherwise.
           `quoted-printable'
                   (Or `qp'.)  Quoted-printable encoding is 7-bit clean and
                   has the property that ASCII characters are passed through
                   unchanged, so that an english message can be read as-is; it
                   is also acceptable for other single-byte locales that share
                   many characters with ASCII, for example ISO-8859-1.  The
                   encoding will cause a large overhead for messages in other
                   character sets: for example it will require up to twelve
                   (12) bytes to encode a single UTF-8 character of four (4)
                   bytes.  It is the default encoding.
           `base64'
                   (Or `b64'.)  This encoding is 7-bit clean and will always
                   be used for binary data.  This encoding has a constant
                   input:output ratio of 3:4, regardless of the character set
                   of the input data it will encode three bytes of input to
                   four bytes of output.  This transfer-encoding is not human
                   readable without performing a decoding step.
     mime-force-sendout
           (Boolean)[Option] Whenever it is not acceptable to fail sending out
           messages because of non-convertible character content this variable
           may be set.  It will, as a last resort, classify the part content
           as `application/octet-stream'.  Please refer to the section
           Character sets for the complete picture of character set conver-
           sion, and HTML mail and MIME attachments for how to internally or
           externally handle part content.
     mimetypes-load-control
           Can be used to control which of The mime.types files are loaded: if
           the letter `u' is part of the option value, then the user's per-
           sonal ~/.mime.types file will be loaded (if it exists); likewise
           the letter `s' controls loading of the system wide /etc/mime.types;
           directives found in the user file take precedence, letter matching
           is case-insensitive.  If this variable is not set S-nail will try
           to load both files.  Incorporation of the S-nail-built-in MIME
           types cannot be suppressed, but they will be matched last (the
           order can be listed via mimetype).
           More sources can be specified by using a different syntax: if the
           value string contains an equals sign `=' then it is instead parsed
           as a comma-separated list of the described letters plus
           `f=FILENAME' pairs; the given filenames will be expanded and
           loaded, and their content may use the extended syntax that is
           described in the section The mime.types files.  Directives found in
           such files always take precedence (are prepended to the MIME type
           cache).
     mta   Select an alternate Mail-Transfer-Agent by either specifying the
           full pathname of an executable (a `file://' prefix may be given),
           or [Option]ally a SMTP aka SUBMISSION protocol URL [v15-compat]:
                 submissions://[user[:password]@]server[:port]
           ([no v15-compat]: `[smtp://]server[:port]'.)  The default has been
           chosen at compile time.  MTA data transfers are always performed in
           asynchronous child processes, and without supervision unless either
           the sendwait or the verbose variable is set.  Also see mta-bcc-ok.
           [Option]ally expansion of aliases(5) can be performed by setting
           mta-aliases.
           For testing purposes there is the `test' pseudo-MTA, which dumps to
           standard output or optionally to a file, and honours
           mbox-fcc-and-pcc:
                 $ echo text | s-nail -:/ -Smta=test -s ubject ex AT am.ple
                 $ </dev/null s-nail -:/ -Smta=test://./xy ex AT am.ple
           For a file-based MTA it may be necessary to set mta-argv0 in in
           order to choose the right target of a modern mailwrapper(8) envi-
           ronment.  It will be passed command line arguments from several
           possible sources: from the variable mta-arguments if set, from the
           command line if given and the variable expandargv allows their use.
           Argument processing of the MTA will be terminated with a -- separa-
           tor.
           The otherwise occurring implicit usage of the following MTA command
           line arguments can be disabled by setting the boolean variable
           mta-no-default-arguments (which will also disable passing -- to the
           MTA): -i (for not treating a line with only a dot `.' character as
           the end of input), -m (shall the variable metoo be set) and -v (if
           the verbose variable is set); in conjunction with the -r command
           line option or r-option-implicit -f as well as possibly -F will
           (not) be passed.
           [Option]ally S-nail can send mail over SMTP aka SUBMISSION network
           connections to a single defined smart host by setting this variable
           to a SMTP or SUBMISSION URL (see On URL syntax and credential
           lookup).  An authentication scheme can be specified via the vari-
           able chain smtp-auth.  Encrypted network connections are
           [Option]ally available, the section Encrypted network communication
           should give an overview and provide links to more information on
           this.  Note that with some mail providers it may be necessary to
           set the smtp-hostname variable in order to use a specific combina-
           tion of from, hostname and mta.  Network communication socket time-
           outs are configurable via socket-connect-timeout.  All generated
           network traffic may be proxied over a SOCKS socks-proxy, it can be
           logged by setting verbose twice.  The following SMTP variants may
           be used:
           o   The plain SMTP protocol (RFC 5321) that normally lives on the
               server port 25 and requires setting the smtp-use-starttls vari-
               able to enter a TLS encrypted session state.  Assign a value
               like [v15-compat] `smtp://[user[:password]@]server[:port]' ([no
               v15-compat] `smtp://server[:port]') to choose this protocol.
           o   The so-called SMTPS which is supposed to live on server port
               465 and is automatically TLS secured.  Unfortunately it never
               became a standardized protocol and may thus not be supported by
               your hosts network service database - in fact the port number
               has already been reassigned to other protocols!
               SMTPS is nonetheless a commonly offered protocol and thus can
               be chosen by assigning a value like [v15-compat]
               `smtps://[user[:password]@]server[:port]' ([no v15-compat]
               `smtps://server[:port]'); due to the mentioned problems it is
               usually necessary to explicitly specify the port as `:465',
               however.
           o   The SUBMISSION protocol (RFC 6409) lives on server port 587 and
               is identically to the SMTP protocol from S-nail's point of
               view; it requires setting smtp-use-starttls to enter a TLS
               secured session state; e.g., [v15-compat]
               `submission://[user[:password]@]server[:port]'.
           o   The SUBMISSIONS protocol (RFC 8314) that lives on server port
               465 and is TLS secured by default.  It can be chosen by assign-
               ing a value like [v15-compat]
               `submissions://[user[:password]@]server[:port]'.  Due to the
               problems mentioned for SMTPS above and the fact that SUBMIS-
               SIONS is new and a successor that lives on the same port as the
               historical engineering mismanagement named SMTPS, it is usually
               necessary to explicitly specify the port as `:465'.
     mta-aliases
           [Option] If set to a path pointing to a text file in valid MTA
           (Postfix) aliases(5) format, the file is loaded and cached (manage-
           able with mtaaliases), and henceforth plain `name' (see expandaddr)
           message receiver names are recursively expanded as a last expansion
           step, after the distribution lists which can be created with alias.
           Constraints on aliases(5) content support: only local addresses
           (names) which are valid usernames (`[a-z_][a-z0-9_-]*[$]?') are
           treated as expandable aliases, and [v15 behaviour may differ]
           `:include:/file/name' directives are not supported.  By including
           `-name' in expandaddr it can be asserted that only expanded names
           (mail addresses) are passed through to the MTA.
     mta-arguments
           Arguments to pass through to a file-based mta (Mail-Transfer-
           Agent), parsed according to Shell-style argument quoting into an
           array of arguments which will be joined onto MTA options from other
           sources, for example `? wysh set mta-arguments='-t -X "/tmp/my
           log"''.
     mta-no-default-arguments
           (Boolean) Avoids passing standard command line options to a file-
           based mta (please see there).
     mta-no-receiver-arguments
           (Boolean) By default all receiver addresses will be passed as com-
           mand line options to a file-based mta.  Setting this variable dis-
           ables this behaviour to aid those MTAs which employ special treat-
           ment of such arguments.  Doing so can make it necessary to pass a
           -t via mta-arguments, to testify the MTA that it should use the
           passed message as a template.
     mta-argv0
           Many systems use a so-called mailwrapper(8) environment to ensure
           compatibility with sendmail(1).  This works by inspecting the name
           that was used to invoke the mail delivery system.  If this variable
           is set then the mailwrapper (the program that is actually executed
           when calling the file-based mta) will treat its contents as that
           name.
     mta-bcc-ok
           (Boolean) In violation of RFC 5322 some MTAs do not remove `Bcc:'
           header lines from transported messages after having noted the
           respective receivers for addressing purposes.  (The MTAs Exim and
           Courier for example require the command line option -t to enforce
           removal.)  Unless this is set corresponding receivers are addressed
           by protocol-specific means or MTA command line options only, the
           header itself is stripped before being sent over the wire.
     netrc-lookup-USER@HOST, netrc-lookup-HOST, netrc-lookup
           (Boolean)[v15-compat][Option] Used to control usage of the user's
           ~/.netrc file for lookup of account credentials, as documented in
           the section On URL syntax and credential lookup and for the command
           netrc; the section The .netrc file documents the file format.  Also
           see netrc-pipe.
     netrc-pipe
           [v15-compat][Option] When ~/.netrc is loaded (see netrc and
           netrc-lookup) then S-nail will read the output of a shell pipe
           instead of the user's ~/.netrc file if this variable is set (to the
           desired shell command).  This can be used to, for example, store ~/
           .netrc in encrypted form: `? set netrc-pipe='gpg -qd
           ~/.netrc.pgp''.
     newfolders
           [Option] If this variable has the value `maildir', newly created
           local folders will be in Maildir instead of MBOX format.
     newmail
           Checks for new mail in the current folder each time the prompt is
           shown.  A Maildir folder must be re-scanned to determine if new
           mail has arrived.  If this variable is set to the special value
           `nopoll' then a Maildir folder will not be rescanned completely,
           but only timestamp changes are detected.  Maildir folders are
           [Option]al.
     outfolder
           (Boolean) Causes a non-absolute filename specified in record, as
           well as the sender-based filenames of the Copy, Save, Followup and
           followup commands to be interpreted relative to the folder direc-
           tory rather than relative to the current directory.
     on-account-cleanup-ACCOUNT, on-account-cleanup
           Macro hook which will be called once an account is left, as the
           very last step before unrolling per-account localopts.  This hook
           is run even in case of fatal errors, including those generated by
           switching to the account as such, and it is advisable to perform
           only absolutely necessary actions, like cleaning up alternates, for
           example.  The specialized form is used in favour of the generic one
           if found.
     on-compose-cleanup
           Macro hook which will be called after the message has been sent (or
           not, in case of failures), as the very last step before unrolling
           compose mode localopts.  This hook is run even in case of fatal
           errors, and it is advisable to perform only absolutely necessary
           actions, like cleaning up alternates, for example.
           For compose mode hooks that may affect the message content please
           see on-compose-enter, on-compose-leave, on-compose-splice.  [v15
           behaviour may differ] This hook exists because alias, alternates,
           commandalias, shortcut, to name a few, are neither covered by
           localopts nor by local: changes applied in compose mode will con-
           tinue to be in effect thereafter.
     on-compose-enter, on-compose-leave
           Macro hooks which will be called once compose mode is entered, and
           after composing has been finished, respectively; the exact order of
           the steps taken is documented for ~., one of the COMMAND ESCAPES.
           Context about the message being worked on can be queried via
           digmsg.  localopts are enabled for these hooks, and changes on
           variables will be forgotten after the message has been sent.
           on-compose-cleanup can be used to perform other necessary cleanup
           steps.
           Here is an example that injects a signature via
           message-inject-tail; instead using on-compose-splice to simply
           inject the file of desire via ~< or ~<! may be a better approach.
                 define t_ocl {
                   vput ! i cat ~/.mysig
                   if $? -eq 0
                      vput csop message-inject-tail trim-end $i
                   end
                   # Alternatively
                   readctl create ~/.mysig
                   if $? -eq 0
                     readall i
                     if $? -eq 0
                       vput csop message-inject-tail trim-end $i
                     end
                     readctl remove ~/.mysig
                   end
                 }
                 set on-compose-leave=t_ocl
     on-compose-splice, on-compose-splice-shell
           These hooks run once the normal compose mode is finished, but
           before the on-compose-leave macro hook is called etc.  Both hooks
           will be executed in a subprocess, with their input and output con-
           nected to S-nail such that they can act as if they would be an
           interactive user.  The difference in between them is that the lat-
           ter is a SHELL command, whereas the former is a normal defined
           macro, but which is restricted to a small set of commands (the
           verbose output of for example list will indicate said capability).
           localopts are enabled for these hooks (in the parent process),
           causing any setting to be forgotten after the message has been
           sent; on-compose-cleanup can be used to perform other cleanup as
           necessary.
           During execution of these hooks S-nail will temporarily forget
           whether it has been started in interactive mode, (a restricted set
           of) COMMAND ESCAPES will always be available, and for guaranteed
           reproducibilities sake escape and ifs will be set to their
           defaults.  The compose mode command ~^ has been especially designed
           for scriptability (via these hooks).  The first line the hook will
           read on its standard input is the protocol version of said command
           escape, currently ``0 0 2'': backward incompatible protocol changes
           have to be expected.
           Care must be taken to avoid deadlocks and other false control flow:
           if both involved processes wait for more input to happen at the
           same time, or one does not expect more input but the other is stuck
           waiting for consumption of its output, etc.  There is no automatic
           synchronization of the hook: it will not be stopped automatically
           just because it, e.g., emits `~x'.  The hooks will however receive
           a termination signal if the parent enters an error condition.  [v15
           behaviour may differ] Protection against and interaction with sig-
           nals is not yet given; it is likely that in the future these
           scripts will be placed in an isolated session, which is signalled
           in its entirety as necessary.
                 define ocs_signature {
                   read version
                   echo '~< ~/.mysig' # '~<! fortune pathtofortunefile'
                 }
                 set on-compose-splice=ocs_signature
                 wysh set on-compose-splice-shell=$'\
                   read version;\
                   printf "hello $version!  Headers: ";\
                   echo \'~^header list\';\
                   read status result;\
                   echo "status=$status result=$result";\
                   '
                 define ocsm {
                   read version
                   echo Splice protocol version is $version
                   echo '~^h l'; read hl; vput csop es subs "${hl}" 0 1
                   if "$es" != 2
                     echoerr 'Cannot read header list'; echo '~x'; xit
                   endif
                   if "$hl" !%?case ' cc'
                     echo '~^h i cc "Diet is your <mirr.or>"'; read es;\
                       vput csop es substring "${es}" 0 1
                     if "$es" != 2
                       echoerr 'Cannot insert Cc: header'; echo '~x'
                       # (no xit, macro finishes anyway)
                     endif
                   endif
                 }
                 set on-compose-splice=ocsm
     on-history-addition
           This hook will be called if an entry is about to be added to the
           history of the MLE, as documented in On terminal control and line
           editor.  It will be called with three arguments: the first is the
           name of the input context (see bind), the second is either an empty
           string or the matching history-gabby type, and the third being the
           complete command line to be added.  The entry will not be added to
           history if the hook uses a non-0 return.  [v15 behaviour may dif-
           fer] A future version will give the expanded command name as the
           third argument, followed by the tokenized command line as parsed in
           the remaining arguments, the first of which is the original unex-
           panded command name; i.e., one may do `shift 4' and will then be
           able to access the positional parameters as usual via *, #, 1 etc.
     on-main-loop-tick
           This hook will be called whenever the program's main event loop is
           about to read the next input line.  Note variable and other changes
           it performs are not scoped as via localopts!
     on-program-exit
           This hook will be called when the program exits, whether via exit
           or quit, or because the send mode is done.  Note: this runs late
           and so terminal settings etc. are already teared down.
     on-resend-cleanup
           [v15 behaviour may differ] Identical to on-compose-cleanup, but is
           only triggered by resend.
     on-resend-enter
           [v15 behaviour may differ] Identical to on-compose-enter, but is
           only triggered by resend; currently there is no digmsg support, for
           example.
     page  (Boolean) If set, each message feed through the command given for
           pipe is followed by a formfeed character `\f'.
     password-USER@HOST, password-HOST, password
           [v15-compat] Variable chain that sets a password, which is used in
           case none has been given in the protocol and account-specific URL;
           as a last resort S-nail will ask for a password on the user's ter-
           minal if the authentication method requires a password.  Specifying
           passwords in a startup file is generally a security risk; the file
           should be readable by the invoking user only.
     password-USER@HOST
           [no v15-compat] (see the chain above for [v15-compat]) Set the
           password for `USER' when connecting to `HOST'.  If no such variable
           is defined for a host, the user will be asked for a password on
           standard input.  Specifying passwords in a startup file is gener-
           ally a security risk; the file should be readable by the invoking
           user only.
     piperaw
           (Boolean) Send messages to the pipe command without performing MIME
           and character set conversions.
     pipe-EXTENSION
           Identical to pipe-TYPE/SUBTYPE except that `EXTENSION' (normalized
           to lowercase using character mappings of the ASCII charset) denotes
           a file extension, for example `xhtml'.  Handlers registered using
           this method take precedence.
     pipe-TYPE/SUBTYPE
           A MIME message part identified as `TYPE/SUBTYPE' (case-insensitive,
           normalized to lowercase using character mappings of the ASCII
           charset) is displayed or quoted, its text is filtered through the
           value of this variable interpreted as a shell command.  Unless
           noted only parts displayable as inline plain text (see
           copiousoutput) are covered, other MIME parts will only be consid-
           ered by and for mimeview.
           The special value question mark `?' forces interpretation of the
           message part as plain text, for example `set
           pipe-application/xml=?'.  (This can also be achieved by adding a
           MIME type-marker via mimetype.)  [Option]ally MIME type handlers
           may be defined via The Mailcap files to which should be referred to
           for documentation of flags like copiousoutput.  Question mark is
           indeed a trigger character to indicate flags that adjust behaviour
           and usage of the rest of the value, the shell command, for example:
                 ? set pipe-X/Y='?!++=? vim ${MAILX_FILENAME_TEMPORARY}'
           `*'   The command output can be reintegrated into this MUA's normal
                 processing: copiousoutput.  Implied when using a plain `'.
           `#'   Only use this handler for display, not for quoting a message:
                 x-mailx-noquote.
           `&'   Run the command asynchronously, do not wait for the handler
                 to exit: x-mailx-async.  The standard output of the command
                 will go to /dev/null.
           `!'   The command must be run on an interactive terminal, the ter-
                 minal will temporarily be released for it to run:
                 needsterminal.
           `+'   Request creation of a zero-sized temporary file, the absolute
                 pathname of which will be made accessible via the environment
                 variable MAILX_FILENAME_TEMPORARY: x-mailx-tmpfile.  If given
                 twice then the file will be unlinked automatically by S-nail
                 when the command loop is entered again at latest:
                 x-mailx-tmpfile-unlink; it is an error to use automatic dele-
                 tion in conjunction with x-mailx-async.
           `='   Normally the MIME part content is passed to the handler via
                 standard input; with this the data will instead be written
                 into MAILX_FILENAME_TEMPORARY (x-mailx-tmpfile-fill), the
                 creation of which is implied; in order to cause automatic
                 deletion of the temporary file two plus signs `++' still have
                 to be used.
           `t'   Text type-marker: display this as normal plain text (for
                 type-markers: The mime.types files).  Identical to only giv-
                 ing plain `?', implies copiousoutput.
           `h'   [Option] HTML type-marker: display via built-in HTML-to-text
                 filter.  Implies copiousoutput.
           `?'   To avoid ambiguities with normal shell command content
                 another question mark can be used to forcefully terminate
                 interpretation of remaining characters.  (Any character not
                 in this list will have the same effect.)
           Some information about the MIME part to be displayed is embedded
           into the environment of the shell command:
           MAILX_CONTENT            The MIME content-type of the part, if
                                    known, the empty string otherwise.
           MAILX_CONTENT_EVIDENCE   If mime-counter-evidence includes the
                                    carry-around-bit (2), then this will be
                                    set to the detected MIME content-type; not
                                    only then identical to MAILX_CONTENT oth-
                                    erwise.
           MAILX_EXTERNAL_BODY_URL  MIME parts of type `message/external-body
                                    access-type=url' will store the access URL
                                    in this variable, it is empty otherwise.
                                    URL targets should not be activated auto-
                                    matically, without supervision.
           MAILX_FILENAME           The filename, if any is set, the empty
                                    string otherwise.
           MAILX_FILENAME_GENERATED
                                    A random string.
           MAILX_FILENAME_TEMPORARY
                                    If temporary file creation has been
                                    requested through the command prefix this
                                    variable will be set and contain the abso-
                                    lute pathname of the temporary file.
     pop3-auth-USER@HOST, pop3-auth-HOST, pop3-auth
           [Option][v15-compat] Variable chain that sets the POP3 authentica-
           tion method.  Supported are the default `plain', [v15-compat]
           `oauthbearer' (see FAQ entry But, how about XOAUTH2 /
           OAUTHBEARER?), as well as [v15-compat] `external' and `externanon'
           for TLS secured connections which pass a client certificate via
           tls-config-pairs.  There may be the [Option]al method [v15-compat]
           `gssapi'.  `externanon' does not need any user credentials,
           `external' and `gssapi' need a user, the remains also require a
           password.  `externanon' solely builds upon the credentials passed
           via a client certificate, and is usually the way to go since tested
           servers do not actually follow RFC 4422, and fail if additional
           credentials are actually passed.  Unless pop3-no-apop is set the
           `plain' method will [Option]ally be replaced with APOP if possible
           (see there).
     pop3-bulk-load-USER@HOST, pop3-bulk-load-HOST, pop3-bulk-load
           (Boolean)[Option] When accessing a POP3 server S-nail loads the
           headers of the messages, and only requests the message bodies on
           user request.  For the POP3 protocol this means that the message
           headers will be downloaded twice.  If this variable is set then
           S-nail will download only complete messages from the given POP3
           server(s) instead.
     pop3-keepalive-USER@HOST, pop3-keepalive-HOST, pop3-keepalive
           [Option] POP3 servers close the connection after a period of inac-
           tivity; the standard requires this to be at least 10 minutes, but
           practical experience may vary.  Setting this variable to a numeric
           value greater than `0' causes a `NOOP' command to be sent each
           value seconds if no other operation is performed.
     pop3-no-apop-USER@HOST, pop3-no-apop-HOST, pop3-no-apop
           (Boolean)[Option] Unless this variable is set the MD5 based `APOP'
           authentication method will be used instead of a chosen `plain'
           pop3-auth when connecting to a POP3 server that advertises support.
           The advantage of `APOP' is that only a single packet is sent for
           the user/password tuple.  (Originally also that the password is not
           sent in clear text over the wire, but for one MD5 does not any
           longer offer sufficient security, and then today transport is
           almost ever TLS secured.)  Note that pop3-no-apop-HOST requires
           [v15-compat].
     pop3-use-starttls-USER@HOST, pop3-use-starttls-HOST, pop3-use-starttls
           (Boolean)[Option] Causes S-nail to issue a `STLS' command to make
           an unencrypted POP3 session TLS encrypted.  This functionality is
           not supported by all servers, and is not used if the session is
           already encrypted by the POP3S method.  Note that
           pop3-use-starttls-HOST requires [v15-compat].
     posix
           (Boolean) This flag enables POSIX mode, which changes behaviour of
           S-nail where that deviates from standardized behaviour.  It is
           automatically squared with the environment variable
           POSIXLY_CORRECT, changing the one will adjust the other.  The fol-
           lowing behaviour is covered and enforced by this mechanism:
           o   In non-interactive mode, any error encountered while loading
               resource files during program startup will cause a program
               exit, whereas in interactive mode such errors will stop loading
               of the currently loaded (stack of) file(s, i.e., recursively).
               These exits can be circumvented on a per-command base by using
               ignerr, one of the Command modifiers, for each command which
               shall be allowed to fail.
           o   alternates will replace the list of alternate addresses instead
               of appending to it.  In addition alternates will only be hon-
               oured for any sort of message reply, and for aliases.
           o   The variable inserting COMMAND ESCAPES ~A, ~a, ~I and ~i will
               expand embedded character sequences `\t' horizontal tabulator
               and `\n' line feed.  [v15 behaviour may differ] For compatibil-
               ity reasons this step will always be performed.
           o   Reading in messages via ~f (COMMAND ESCAPES) will use the
               `type' not the `forward' headerpick selection.
           o   Upon changing the active folder no summary of headers will be
               displayed even if header is set.
           o   Setting ignoreeof implies the behaviour described by dot.
           o   The variable keep is extended to cover any empty mailbox, not
               only empty primary system mailboxes: they will be removed when
               they are left in empty state otherwise.
           o   Each command has an exit ? and error ! status that overwrites
               that of the last command.  In POSIX mode the program exit sta-
               tus will signal failure regardless unless all messages were
               successfully sent out to the mta; also see sendwait.
     print-alternatives
           (Boolean) When a MIME message part of type `multipart/alternative'
           is displayed and it contains a subpart of type `text/plain', other
           parts are normally discarded.  Setting this variable causes all
           subparts to be displayed, just as if the surrounding part was of
           type `multipart/mixed'.
     prompt
           The string used as a prompt in interactive mode.  Whenever the
           variable is evaluated the value is treated as if specified within
           dollar-single-quotes (see Shell-style argument quoting).  This
           (post-assignment, i.e., second) expansion can be used to embed sta-
           tus information, for example ?, !, account or mailbox-display.
           In order to embed characters which should not be counted when cal-
           culating the visual width of the resulting string, enclose the
           characters of interest in a pair of reverse solidus escaped brack-
           ets: `\[\E[0m\]'; a slot for coloured prompts is also available
           with the [Option]al command colour.  Prompting may be prevented by
           setting this to the null string (aka `set noprompt').
     prompt2
           This string is used for secondary prompts, but is otherwise identi-
           cal to prompt.  The default is `.. '.
     quiet
           (Boolean) Suppresses the printing of the version when first
           invoked.
     quote
           If set messages processed by variants of followup and reply will
           start with the original message, lines of which prefixed by
           indentprefix, taking into account quote-chars and quote-fold.  No
           headers will be quoted when set without value or for `noheading',
           for `headers' the `type' headerpick selection will be included in
           the quote, `allbodies' embeds the (body) contents of all MIME
           parts, and `allheaders' also includes all headers.  The quoted mes-
           sage will be enclosed by the expansions of quote-inject-head and
           quote-inject-tail.  Also see quote-add-cc, quote-as-attachment and
           ~Q, one of the COMMAND ESCAPES.
     quote-add-cc
           (Boolean) Whether senders of messages quoted via ~Q shall be made
           members of the carbon copies `Cc:' list.
     quote-as-attachment
           (Boolean) Add the original message in its entirety as a
           `message/rfc822' MIME attachment when replying to a message.  Note
           this works regardless of the setting of quote.
     quote-chars
           Can be set to a string consisting of non-whitespace ASCII charac-
           ters which shall be treated as quotation leaders, the default being
           `>|}:'.
     quote-fold
           [Option] Can be set in addition to indentprefix, and creates a more
           fancy quotation in that leading quotation characters (quote-chars)
           are compressed and overlong lines are folded.  quote-fold can be
           set to either one, two or three (space separated) numeric values,
           which are interpreted as the maximum (goal) and the minimum line
           length, respectively, in a spirit rather equal to the fmt(1) pro-
           gram, but line- instead of paragraph-based.  The third value is
           used as the maximum line length instead of the first if no better
           break point can be found; it is ignored unless it is larger than
           the minimum and smaller than the maximum.  If not set explicitly
           the minimum will reflect the goal algorithmically.  The goal cannot
           be smaller than the length of indentprefix plus some additional
           pad; necessary adjustments take place silently.
     quote-inject-head, quote-inject-tail
           The strings to put before and after the text of a quoted message,
           if non-empty, and respectively.  The former defaults to `%f
           wrote:\n\n'.  Special format directives will be expanded if possi-
           ble, and if so configured the output will be folded according to
           quote-fold.  Format specifiers in the given strings start with a
           percent sign `%' and expand values of the original message, unless
           noted otherwise.  Note that names and addresses are not subject to
           the setting of showto.  Valid format specifiers are:
           `%%'    A plain percent sign.
           `%a'    The address(es) of the sender(s).
           `%d'    The date found in the `Date:' header of the message when
                   datefield is set (the default), otherwise the date when the
                   message was received.  Formatting can be controlled by
                   assigning a strftime(3) format string to datefield (and
                   datefield-markout-older).
           `%f'    The full name(s) (name and address, as given) of the
                   sender(s).
           `%i'    The `Message-ID:'.
           `%n'    The real name(s) of the sender(s) if there is one and
                   showname allows usage, the address(es) otherwise.
           `%r'    The senders real name(s) if there is one, the address(es)
                   otherwise.
     r-option-implicit
           (Boolean) Setting this option evaluates the contents of from (or,
           if that contains multiple addresses, sender) and passes the results
           onto the used (file-based) MTA as described for the -r option
           (empty argument case).
     recipients-in-cc
           (Boolean) When doing a reply, the original `From:' and `To:' as
           well as addressees which possibly came in via `Reply-To:' and
           `Mail-Followup-To:' are by default merged into the new `To:'.  If
           this variable is set a sensitive algorithm tries to place in `To:'
           only the sender of the message being replied to, others are placed
           in `Cc:'.
     record
           Unless this variable is defined, no copies of outgoing mail will be
           saved.  If defined it gives the pathname, subject to the usual
           Filename transformations, of a folder where all new, replied-to or
           forwarded messages are saved: when saving to this folder fails the
           message is not sent, but instead saved to DEAD.  The standard
           defines that relative (fully expanded) paths are to be interpreted
           relative to the current directory (cwd), to force interpretation
           relative to folder outfolder needs to be set in addition.
     record-files
           (Boolean) If this variable is set the meaning of record will be
           extended to cover messages which target only file and pipe recipi-
           ents (see expandaddr).  These address types will not appear in
           recipient lists unless add-file-recipients is also set.
     record-resent
           (Boolean) If this variable is set the meaning of record will be
           extended to also cover the resend and Resend commands.
     reply-in-same-charset
           (Boolean) If this variable is set S-nail first tries to use the
           same character set of the original message for replies.  If this
           fails, the mechanism described in Character sets is evaluated as
           usual.
     reply-strings
           Can be set to a comma-separated list of (case-insensitive according
           to ASCII rules) strings which shall be recognized in addition to
           the built-in strings as `Subject:' reply message indicators -
           built-in are `Re:', which is mandated by RFC 5322, as well as the
           german `Aw:', `Antw:', and the `Wg:' which often has been seen in
           the wild; I.e., the separating colon has to be specified explic-
           itly.
     reply-to
           A list of addresses to put into the `Reply-To:' field of the mes-
           sage header.  Members of this list are handled as if they were in
           the alternates list.
     replyto
           [Obsolete] Variant of reply-to.
     reply-to-honour
           Controls whether a `Reply-To:' header is honoured when replying to
           a message via reply or Lreply.  This is a quadoption; if set with-
           out a value it defaults to ``yes''.
     reply-to-swap-in
           Standards like DKIM and (in conjunction with) DMARC caused many
           Mailing lists to use sender address rewriting in the style of `Name
           via List <list@address>', where the original sender address often
           being placed in `Reply-To:'.  If this is set and a `Reply-To:'
           exists, and consists of only one addressee (!), then that is used
           in place of the pretended sender.  This works independently from
           reply-to-honour.  The optional value, a comma-separated list of
           strings, offers more fine-grained control on when swapping shall be
           used; for now supported is mlist, here swapping occurs if the
           sender is a mailing-list as defined by mlist.
     rfc822-body-from_
           (Boolean) This variable can be used to force displaying a so-called
           `From_' line for messages that are embedded into an envelope mail
           via the `message/rfc822' MIME mechanism, for more visual conve-
           nience, also see mbox-rfc4155.
     save  (Boolean) Enable saving of (partial) messages in DEAD upon inter-
           rupt or delivery error.
     screen
           The number of lines that represents a ``screenful'' of lines, used
           in headers summary display, from searching, message topline display
           and scrolling via z.  If this variable is not set S-nail falls back
           to a calculation based upon the detected terminal window size and
           the baud rate: the faster the terminal, the more will be shown.
           Overall screen dimensions and pager usage is influenced by the
           environment variables COLUMNS and LINES and the variable crt.
     searchheaders
           (Boolean) Expand message list specifiers in the form `/x:y' to all
           messages containing the substring ``y'' in the header field `x'.
           The string search is case insensitive.
     sendcharsets
           [Option] A comma-separated list of character set names that can be
           used in outgoing internet mail.  The value of the variable
           charset-8bit is automatically appended to this list of character
           sets.  If no character set conversion capabilities are compiled
           into S-nail then the only supported charset is ttycharset.  Also
           see sendcharsets-else-ttycharset and refer to the section Character
           sets for the complete picture of character set conversion in
           S-nail.
     sendcharsets-else-ttycharset
           (Boolean)[Option] If this variable is set, but sendcharsets is not,
           then S-nail acts as if sendcharsets had been set to the value of
           the variable ttycharset.  In effect this combination passes through
           the message data in the character set of the current locale encod-
           ing: therefore mail message text will be (assumed to be) in
           ISO-8859-1 encoding when send from within a ISO-8859-1 locale, and
           in UTF-8 encoding when send from within an UTF-8 locale.
           The 8-bit fallback charset-8bit never comes into play as ttycharset
           is implicitly assumed to be 8-bit and capable to represent all
           files the user may specify (as is the case when no character set
           conversion support is available in S-nail and the only supported
           character set is ttycharset, see Character sets).  This might be a
           problem for scripts which use the suggested `LC_ALL=C' setting,
           since in this case the character set is US-ASCII by definition, so
           that it is better to also override ttycharset, then; and/or do
           something like the following in the resource file:
                 # Avoid ASCII "propagates to 8-bit" when scripting
                 \if ! t && "$LC_ALL" != C && "$LC_CTYPE" != C
                   \set sendcharsets-else-ttycharset
                 \end
     sender
           An address that is put into the `Sender:' field of outgoing mes-
           sages, quoting RFC 5322: the mailbox of the agent responsible for
           the actual transmission of the message.  This field should normally
           not be used unless the from field contains more than one address,
           on which case it is required.  [v15 behaviour may differ] Please
           expect automatic management of the from and sender relationship.
           Dependent on the context this address is handled as if it were in
           the list of alternates.  Also see -r, r-option-implicit.
     sendmail
           [Obsolete] Predecessor of mta.
     sendmail-arguments
           [Obsolete] Predecessor of mta-arguments.
     sendmail-no-default-arguments
           [Obsolete](Boolean) Predecessor of mta-no-default-arguments.
     sendmail-progname
           [Obsolete] Predecessor of mta-argv0.
     sendwait
           Sending messages to the chosen mta or to command-pipe receivers
           (see On sending mail, and non-interactive mode) will be performed
           asynchronously.  This means that only startup errors of the respec-
           tive program will be recognizable, but no delivery errors.  Also,
           no guarantees can be made as to when the respective program will
           actually run, as well as to when they will have produced output.
           If this variable is set then child program exit is waited for, and
           its exit status code is used to decide about success.  Remarks: in
           conflict with the POSIX standard this variable is built-in to be
           initially set.  Another difference is that it can have a value,
           which is interpreted as a comma-separated list of case-insensitive
           strings naming specific subsystems for which synchronousness shall
           be ensured (only).  Possible values are `mta' for mta delivery, and
           `pcc' for command-pipe receivers.
     showlast
           (Boolean) This setting causes S-nail to start at the last message
           instead of the first one when opening a mail folder, as well as
           with from and headers.
     showname
           (Boolean) Causes S-nail to use the sender's real name instead of
           the plain address in the header field summary and in message speci-
           fications.
     showto
           (Boolean) Causes the recipient of the message to be shown in the
           header summary if the message was sent by the user.
     Sign  The value backing ~A, one of the COMMAND ESCAPES.  Also see
           message-inject-tail, on-compose-leave and on-compose-splice.
     sign  The value backing ~a, one of the COMMAND ESCAPES.  Also see
           message-inject-tail, on-compose-leave and on-compose-splice.
     signature
           [Obsolete] Please use on-compose-splice or on-compose-splice-shell
           or on-compose-leave and (if necessary) message-inject-tail instead!
     skipemptybody
           (Boolean) If an outgoing message has an empty first or only message
           part, do not send, but discard it, successfully (also see the com-
           mand line option -E).
     smime-ca-dir, smime-ca-file
           [Option] Specify the location of trusted CA certificates in PEM
           (Privacy Enhanced Mail) for the purpose of verification of S/MIME
           signed messages.  tls-ca-dir documents the necessary preparation
           steps to use the former.  The set of CA certificates which are
           built into the TLS library can be explicitly turned off by setting
           smime-ca-no-defaults, and further fine-tuning is possible via
           smime-ca-flags.
     smime-ca-flags
           [Option] Can be used to fine-tune behaviour of the X509 CA certifi-
           cate storage, and the certificate verification that is used.  The
           actual values and their meanings are documented for tls-ca-flags.
     smime-ca-no-defaults
           (Boolean)[Option] Do not load the default CA locations that are
           built into the used to TLS library to verify S/MIME signed mes-
           sages.
     smime-cipher-USER@HOST, smime-cipher
           [Option] Specifies the cipher to use when generating S/MIME
           encrypted messages (for the specified account).  RFC 5751 mandates
           a default of `aes128' (AES-128 CBC).  Possible values are (case-
           insensitive and) in decreasing cipher strength: `aes256' (AES-256
           CBC), `aes192' (AES-192 CBC), `aes128' (AES-128 CBC), `des3' (DES
           EDE3 CBC, 168 bits; default if `aes128' is not available) and `des'
           (DES CBC, 56 bits).
           The actually available cipher algorithms depend on the crypto-
           graphic library that S-nail uses.  [Option] Support for more cipher
           algorithms may be available through dynamic loading via
           EVP_get_cipherbyname(3) (OpenSSL) if S-nail has been compiled to
           support this.
     smime-crl-dir
           [Option] Specifies a directory that contains files with CRLs in PEM
           format to use when verifying S/MIME messages.
     smime-crl-file
           [Option] Specifies a file that contains a CRL in PEM format to use
           when verifying S/MIME messages.
     smime-encrypt-USER@HOST
           [Option] If this variable is set, messages send to the given
           receiver are encrypted before sending.  The value of the variable
           must be set to the name of a file that contains a certificate in
           PEM format.
           If a message is sent to multiple recipients, each of them for whom
           a corresponding variable is set will receive an individually
           encrypted message; other recipients will continue to receive the
           message in plain text unless the smime-force-encryption variable is
           set.  It is recommended to sign encrypted messages, i.e., to also
           set the smime-sign variable.  content-description-smime-message
           will be inspected for messages which become encrypted.
     smime-force-encryption
           (Boolean)[Option] Causes S-nail to refuse sending unencrypted mes-
           sages.
     smime-sign
           (Boolean)[Option] S/MIME sign outgoing messages with the user's
           (from) private key and include the users certificate as a MIME
           attachment.  Signing a message enables a recipient to verify that
           the sender used a valid certificate, that the email addresses in
           the certificate match those in the message header and that the mes-
           sage content has not been altered.  It does not change the message
           text, and people will be able to read the message as usual.
           content-description-smime-signature will be inspected.  Also see
           smime-sign-cert, smime-sign-include-certs and smime-sign-digest.
     smime-sign-cert-USER@HOST, smime-sign-cert
           [Option] Points to a file in PEM format.  For the purpose of sign-
           ing and decryption this file needs to contain the user's private
           key, followed by his certificate.
           For message signing `USER@HOST' is always derived from the value of
           from (or, if that contains multiple addresses, sender).  For the
           purpose of encryption the recipients public encryption key (cer-
           tificate) is expected; the command certsave can be used to save
           certificates of signed messages (the section Signed and encrypted
           messages with S/MIME gives some details).  This mode of operation
           is usually driven by the specialized form.
           When decrypting messages the account is derived from the recipient
           fields (`To:' and `Cc:') of the message, which are searched for
           addresses for which such a variable is set.  S-nail always uses the
           first address that matches, so if the same message is sent to more
           than one of the user addresses using different encryption keys,
           decryption might fail.
           Password-encrypted keys may be used for signing and decryption.
           Automated password lookup is possible via the ``pseudo-hosts''
           `USER AT HOST.smime-cert-key' for the private key, and
           `USER AT HOST.smime-cert-cert' for the certificate stored in the same
           file.  For example, the hypothetical address `bob AT exam.ple' could
           be driven with a private key / certificate pair path defined in
           smime-sign-cert-bob AT exam.ple, and the needed passwords would then
           be looked up as `bob AT exam.smime-cert-key' and
           `bob AT exam.smime-cert-cert'.  When decrypting the value of from
           will be tried as a fallback to provide the necessary `USER@HOST'.
           To include intermediate certificates, use smime-sign-include-certs.
           The possible password sources are documented in On URL syntax and
           credential lookup.
     smime-sign-digest-USER@HOST, smime-sign-digest
           [Option] Specifies the message digest to use when signing S/MIME
           messages.  Please remember that for this use case `USER@HOST'
           refers to the variable from (or, if that contains multiple
           addresses, sender).  The available algorithms depend on the used
           cryptographic library, but at least one usable built-in algorithm
           is ensured as a default.  If possible the standard RFC 5751 will be
           violated by using `SHA512' instead of the mandated `SHA1' due to
           security concerns.  This variable is ignored for very old (released
           before 2010) cryptographic libraries which do not offer the neces-
           sary interface: it will be logged if that happened.
           S-nail will try to add built-in support for the following message
           digests, names are case-insensitive: `BLAKE2b512', `BLAKE2s256',
           `SHA3-512', `SHA3-384', `SHA3-256', `SHA3-224', as well as the
           widely available `SHA512', `SHA384', `SHA256', `SHA224', and the
           proposed insecure `SHA1', finally `MD5'.  More digests may
           [Option]ally be available through dynamic loading via the OpenSSL
           function EVP_get_digestbyname(3).
     smime-sign-include-certs-USER@HOST, smime-sign-include-certs
           [Option] If used, this is supposed to a consist of a comma-sepa-
           rated list of files, each of which containing a single certificate
           in PEM format to be included in the S/MIME message in addition to
           the smime-sign-cert certificate.  This can be used to include
           intermediate certificates of the certificate authority, in order to
           allow the receiver's S/MIME implementation to perform a verifica-
           tion of the entire certificate chain, starting from a local root
           certificate, over the intermediate certificates, down to the
           smime-sign-cert.  Even though top level certificates may also be
           included in the chain, they will not be used for the verification
           on the receiver's side.
           For the purpose of the mechanisms involved here, `USER@HOST' refers
           to the content of the internal variable from (or, if that contains
           multiple addresses, sender).  The pseudo-host
           `USER AT HOST.smime-include-certs' will be used for performing pass-
           word lookups for these certificates, shall they have been given
           one, therefore the lookup can be automated via the mechanisms
           described in On URL syntax and credential lookup.
     smime-sign-message-digest-USER@HOST, smime-sign-message-digest
           [Obsolete][Option] Predecessor(s) of smime-sign-digest.
     smtp  [Obsolete][Option] To use the built-in SMTP transport, specify a
           SMTP URL in mta.  [v15 behaviour may differ] For compatibility rea-
           sons a set smtp is used in preference of mta.
     smtp-auth-USER@HOST, smtp-auth-HOST, smtp-auth
           [Option] Variable chain that controls the SMTP mta authentication
           method, possible values are `none' ([no v15-compat] default),
           `plain' ([v15-compat] default), `login', [v15-compat] `oauthbearer'
           (see FAQ entry But, how about XOAUTH2 / OAUTHBEARER?) as well as
           [v15-compat] `external' and `externanon' for TLS secured connec-
           tions which pass a client certificate via tls-config-pairs.  There
           may be the [Option]al methods `cram-md5' and `gssapi'.  `none' and
           `externanon' do not need any user credentials, `external' and
           `gssapi' require a user name, and all other methods require a user
           name and a password.  `externanon' solely builds upon the creden-
           tials passed via a client certificate, and is usually the way to go
           since tested servers do not actually follow RFC 4422 aka RFC 4954,
           and fail if additional credentials are passed.  Also see mta.  Note
           that smtp-auth-HOST is [v15-compat].  ([no v15-compat] Requires
           smtp-auth-password and smtp-auth-user.  Note for
           smtp-auth-USER@HOST: may override dependent on sender address in
           the variable from.)
     smtp-auth-password
           [Option][no v15-compat] Sets the global fallback password for SMTP
           authentication.  If the authentication method requires a password,
           but neither smtp-auth-password nor a matching
           smtp-auth-password-USER@HOST can be found, S-nail will ask for a
           password on the user's terminal.
     smtp-auth-password-USER@HOST
           [no v15-compat] Overrides smtp-auth-password for specific values of
           sender addresses, dependent upon the variable from.
     smtp-auth-user
           [Option][no v15-compat] Sets the global fallback user name for SMTP
           authentication.  If the authentication method requires a user name,
           but neither smtp-auth-user nor a matching smtp-auth-user-USER@HOST
           can be found, S-nail will ask for a user name on the user's termi-
           nal.
     smtp-auth-user-USER@HOST
           [no v15-compat] Overrides smtp-auth-user for specific values of
           sender addresses, dependent upon the variable from.
     smtp-hostname
           [Option][v15-compat] Normally S-nail uses the variable from to
           derive the necessary `USER@HOST' information in order to issue a
           `MAIL FROM:<>' SMTP mta command.  Setting smtp-hostname can be used
           to use the `USER' from the SMTP account (mta or the user variable
           chain) and the given `HOST' (hostname if the empty string is given,
           or the local hostname as a last resort).  This often allows using
           an address that is itself valid but hosted by a provider other than
           from which (in from) the message is sent.  Setting this variable
           also influences generated `Message-ID:' and `Content-ID:' header
           fields.  If the [Option]al IDNA support is available (see
           idna-disable) variable assignment is aborted when a necessary con-
           version fails.
     smtp-use-starttls-USER@HOST, smtp-use-starttls-HOST, smtp-use-starttls
           (Boolean)[Option] Causes S-nail to issue a `STARTTLS' command to
           make an SMTP mta session TLS encrypted, i.e., to enable transport
           layer security.
     socket-connect-timeout
           [Option] A positive number that defines the timeout to wait for
           establishing a socket connection before forcing ^ERR-TIMEDOUT.
     socks-proxy-USER@HOST, socks-proxy-HOST, socks-proxy
           [Option] If set to the URL of a SOCKS5 server then all network
           activities are proxied through it, except for the single DNS name
           lookup necessary to resolve the proxy URL (unnecessary when given
           an already resolved IP address).  It is automatically squared with
           the environment variable SOCKS5_PROXY, changing the one will adjust
           the other.  This example creates a local SOCKS5 proxy on port 10000
           that forwards to the machine `HOST' (with identity `USER'), and
           from which actual network traffic happens:
                 $ ssh -D 10000 USER@HOST
                 $ s-nail -Ssocks-proxy=[socks5://]localhost:10000
                 # or =localhost:10000; no local DNS: =127.0.0.1:10000
     spam-interface
           [Option] In order to use any of the spam-related commands (like
           spamrate) the desired spam interface must be defined by setting
           this variable.  Please refer to the manual section Handling spam
           for the complete picture of spam handling in S-nail.  All or none
           of the following interfaces may be available:
           `spamc'   Interaction with spamc(1) from the spamassassin(1)
                     (http://spamassassin.apache.org SpamAssassin) suite.
                     Different to the generic filter interface S-nail will
                     automatically add the correct arguments for a given com-
                     mand and has the necessary knowledge to parse the pro-
                     gram's output.  A default value for spamc-command will
                     have been compiled into the S-nail binary if spamc(1) has
                     been found in PATH during compilation.  Shall it be nec-
                     essary to define a specific connection type (rather than
                     using a configuration file for that), the variable
                     spamc-arguments can be used as in for example `-d
                     server.example.com -p 783'.  It is also possible to spec-
                     ify a per-user configuration via spamc-user.  Note that
                     this interface does not inspect the `is-spam' flag of a
                     message for the command spamforget.
           `filter'  generic spam filter support via freely configurable
                     hooks.  This interface is meant for programs like
                     bogofilter(1) and requires according behaviour in respect
                     to the hooks' exit status for at least the command
                     spamrate (`0' meaning a message is spam, `1' for non-
                     spam, `2' for unsure and any other return value indicat-
                     ing a hard error); since the hooks can include shell code
                     snippets diverting behaviour can be intercepted as neces-
                     sary.  The hooks are spamfilter-ham, spamfilter-noham,
                     spamfilter-nospam, spamfilter-rate and spamfilter-spam;
                     the manual section Handling spam contains examples for
                     some programs.  The process environment of the hooks will
                     have the variable MAILX_FILENAME_GENERATED set.  Note
                     that spam score support for spamrate is not supported
                     unless the [Option]tional regular expression support is
                     available and the spamfilter-rate-scanscore variable is
                     set.
     spam-maxsize
           [Option] Messages that exceed this size will not be passed through
           to the configured spam-interface.  If unset or 0, the default of
           420000 bytes is used.
     spamc-command
           [Option] The path to the spamc(1) program for the `spamc'
           spam-interface.  Note that the path is not expanded, but used ``as
           is''.  A fallback path will have been compiled into the S-nail
           binary if the executable had been found during compilation.
     spamc-arguments
           [Option] Even though S-nail deals with most arguments for the
           `spamc' spam-interface automatically, it may at least sometimes be
           desirable to specify connection-related ones via this variable, for
           example `-d server.example.com -p 783'.
     spamc-user
           [Option] Specify a username for per-user configuration files for
           the `spamc' spam-interface.  If this is set to the empty string
           then S-nail will use the name of the current user.
     spamfilter-ham, spamfilter-noham, spamfilter-nospam, spamfilter-rate,
           spamfilter-spam
           [Option] Command and argument hooks for the `filter'
           spam-interface.  The manual section Handling spam contains examples
           for some programs.
     spamfilter-rate-scanscore
           [Option] Because of the generic nature of the `filter'
           spam-interface spam scores are not supported for it by default, but
           if the [Option]nal regular expression support is available then
           setting this variable can be used to overcome this restriction.  It
           is interpreted as follows: first a number (digits) is parsed that
           must be followed by a semicolon `;' and an extended regular expres-
           sion.  Then the latter is used to parse the first output line of
           the spamfilter-rate hook, and, in case the evaluation is success-
           ful, the group that has been specified via the number is inter-
           preted as a floating point scan score.
     ssl-ca-dir-USER@HOST, ssl-ca-dir-HOST, ssl-ca-dir, ssl-ca-file-USER@HOST,
           ssl-ca-file-HOST, ssl-ca-file
           [Obsolete][Option] Predecessors of tls-ca-file, tls-ca-dir.
     ssl-ca-flags-USER@HOST, ssl-ca-flags-HOST, ssl-ca-flags
           [Obsolete][Option] Predecessor of tls-ca-flags.
     ssl-ca-no-defaults-USER@HOST, ssl-ca-no-defaults-HOST, ssl-ca-no-defaults
           [Obsolete](Boolean)[Option] Predecessor of tls-ca-no-defaults.
     ssl-cert-USER@HOST, ssl-cert-HOST, ssl-cert
           [Obsolete][Option] Please use the Certificate slot of
           tls-config-pairs.
     ssl-cipher-list-USER@HOST, ssl-cipher-list-HOST, ssl-cipher-list
           [Obsolete][Option] Please use the CipherString slot of
           tls-config-pairs.
     ssl-config-file
           [Obsolete][Option] Predecessor of tls-config-file.
     ssl-config-module-USER@HOST, ssl-config-module-HOST, ssl-config-module
           [Obsolete][Option] Predecessor of tls-config-module.
     ssl-config-pairs-USER@HOST, ssl-config-pairs-HOST, ssl-config-pairs
           [Obsolete][Option] Predecessor of tls-config-pairs.
     ssl-crl-dir, ssl-crl-file
           [Obsolete][Option] Predecessors of tls-crl-dir, tls-crl-file.
     ssl-curves-USER@HOST, ssl-curves-HOST, ssl-curves
           [Obsolete][Option] Please use the Curves slot of tls-config-pairs.
     ssl-features
           [Obsolete][Option](Read-only) Predecessor of tls-features.
     ssl-key-USER@HOST, ssl-key-HOST, ssl-key
           [Obsolete][Option] Please use the PrivateKey slot of
           tls-config-pairs.
     ssl-method-USER@HOST, ssl-method-HOST, ssl-method
           [Obsolete][Option] Please use the Protocol slot of
           tls-config-pairs.
     ssl-protocol-USER@HOST, ssl-protocol-HOST, ssl-protocol
           [Obsolete][Option] Please use the Protocol slot of
           tls-config-pairs.
     ssl-rand-file
           [Obsolete][Option] Predecessor of tls-rand-file.
     ssl-verify-USER@HOST, ssl-verify-HOST, ssl-verify
           [Obsolete][Option] Predecessor of tls-verify.
     stealthmua
           If only set without an assigned value, then this setting inhibits
           the generation of the `Message-ID:', `Content-ID:' and
           `User-Agent:' header fields that include obvious references to
           S-nail.  There are two pitfalls associated with this: First, the
           message id of outgoing messages is not known anymore.  Second, an
           expert may still use the remaining information in the header to
           track down the originating mail user agent.  If set to the value
           `noagent', then the mentioned `Message-ID:' and `Content-ID:' sup-
           pression does not occur.
     system-mailrc
           (Read-only) The compiled in path of the system wide initialization
           file one of the Resource files: s-nail.rc.
     termcap
           ([Option]) This specifies a comma-separated list of Terminal
           Information Library (libterminfo, -lterminfo) and/or Termcap Access
           Library (libtermcap, -ltermcap) capabilities (see On terminal
           control and line editor, escape commas with reverse solidus `\') to
           be used to overwrite or define entries.  Note this variable will
           only be queried once at program startup and can thus only be speci-
           fied in resource files or on the command line.  It will always be
           inspected, regardless of whether features denotes termcap/terminfo
           library support via `,+termcap,'.
           String capabilities form `cap=value' pairs and are expected unless
           noted otherwise.  Numerics have to be notated as `cap#number' where
           the number is expected in normal decimal notation.  Finally, bool-
           eans do not have any value but indicate a true or false state sim-
           ply by being defined or not; this indeed means that S-nail does not
           support undefining an existing boolean.  String capability values
           will undergo some expansions before use: for one notations like
           `^LETTER' stand for `control-LETTER', and for clarification pur-
           poses `\E' can be used to specify `escape' (the control notation
           `^[' could lead to misreadings when a left bracket follows, which
           it does for the standard CSI sequence); finally three letter octal
           sequences, as in `\061', are supported.  To specify that a terminal
           supports 256-colours, and to define sequences that home the cursor
           and produce an audible bell, one might write:
                 ? set termcap='Co#256,home=\E[H,bel=^G'
           The following terminal capabilities are or may be meaningful for
           the operation of the built-in line editor or S-nail in general:
           am   auto_right_margin: boolean which indicates if the right margin
                needs special treatment; the xenl capability is related, for
                more see COLUMNS.  This capability is only used when backed by
                library support.
           clear or cl
                clear_screen: clear the screen and home cursor.  (Will be sim-
                ulated via ho plus cd.)
           colors or Co
                max_colors: numeric capability specifying the maximum number
                of colours.  Note that S-nail does not actually care about the
                terminal beside that, but always emits ANSI / ISO 6429 escape
                sequences; also see colour.
           cr   carriage_return: move to the first column in the current row.
                The default built-in fallback is `\r'.
           cub1 or le
                cursor_left: move the cursor left one space (non-destruc-
                tively).  The default built-in fallback is `\b'.
           cuf1 or nd
                cursor_right: move the cursor right one space (non-destruc-
                tively).  The default built-in fallback is `\E[C', which is
                used by most terminals.  Less often occur `\EC' and `\EOC'.
           ed or cd
                clr_eos: clear the screen.
           el or ce
                clr_eol: clear to the end of line.  (Will be simulated via ch
                plus repetitions of space characters.)
           home or ho
                cursor_home: home cursor.
           hpa or ch
                column_address: move the cursor (to the given column parame-
                ter) in the current row.  (Will be simulated via cr plus nd.)
           rmcup or te / smcup or ti
                exit_ca_mode and enter_ca_mode, respectively: exit and enter
                the alternative screen ca-mode, effectively turning S-nail
                into a fullscreen application.  This must be enabled explic-
                itly by setting termcap-ca-mode.
           smkx or ks / rmkx or ke
                keypad_xmit and keypad_local, respectively: enable and disable
                the keypad.  This is always enabled if available, because it
                seems even keyboards without keypads generate other key codes
                for, e.g., cursor keys in that case, and only if enabled we
                see the codes that we are interested in.
           xenl or xn
                eat_newline_glitch: boolean which indicates whether a newline
                written in the last column of an auto_right_margin indicating
                terminal is ignored.  With it the full terminal width is
                available even on autowrap terminals.  This will be inspected
                even without `,+termcap,' features.
           Many more capabilities which describe key-sequences are documented
           for bind.
     termcap-ca-mode
           [Option] Allow usage of the exit_ca_mode and enter_ca_mode
           termcapabilities in order to enter an alternative exclusive screen,
           the so-called ca-mode; this usually requires special configuration
           of the PAGER, also dependent on the value of crt.  Note this vari-
           able will only be queried once at program startup and can thus only
           be specified in resource files or on the command line.
     termcap-disable
           [Option] Disable any interaction with a terminal control library.
           If set only some generic fallback built-ins and possibly the con-
           tent of termcap describe the terminal to S-nail.  Note this vari-
           able will only be queried once at program startup and can thus only
           be specified in resource files or on the command line.
     tls-ca-dir-USER@HOST, tls-ca-dir-HOST, tls-ca-dir, tls-ca-file-USER@HOST,
           tls-ca-file-HOST, tls-ca-file
           [Option] Directory and file, respectively, for pools of trusted CA
           certificates in PEM (Privacy Enhanced Mail) format, for the purpose
           of verification of TLS server certificates.  Concurrent use is pos-
           sible, the file is loaded once needed first, the directory lookup
           is performed anew as a last resort whenever necessary.  The CA cer-
           tificate pool built into the TLS library can be disabled via
           tls-ca-no-defaults, further fine-tuning is possible via
           tls-ca-flags.  The directory search requires special filename con-
           ventions, please see SSL_CTX_load_verify_locations(3) and verify(1)
           (or c_rehash(1)).
     tls-ca-flags-USER@HOST, tls-ca-flags-HOST, tls-ca-flags
           [Option] Can be used to fine-tune behaviour of the X509 CA certifi-
           cate storage, and the certificate verification that is used (also
           see tls-verify).  The value is expected to consist of a comma-sepa-
           rated list of configuration directives, with any intervening white-
           space being ignored.  The directives directly map to flags that can
           be passed to X509_STORE_set_flags(3), which are usually defined in
           a file openssl/x509_vfy.h, and the availability of which depends on
           the used TLS library version: a directive without mapping is
           ignored (error log subject to debug).  Directives currently under-
           stood (case-insensitively) include:
           no-alt-chains
                 If the initial chain is not trusted, do not attempt to build
                 an alternative chain.  Setting this flag will make OpenSSL
                 certificate verification match that of older OpenSSL ver-
                 sions, before automatic building and checking of alternative
                 chains has been implemented; also see trusted-first.
           no-check-time
                 Do not check certificate/CRL validity against current time.
           partial-chain
                 By default partial, incomplete chains which cannot be veri-
                 fied up to the chain top, a self-signed root certificate,
                 will not verify.  With this flag set, a chain succeeds to
                 verify if at least one signing certificate of the chain is in
                 any of the configured trusted stores of CA certificates.  The
                 OpenSSL manual page SSL_CTX_load_verify_locations(3) gives
                 some advise how to manage your own trusted store of CA cer-
                 tificates.
           strict
                 Disable workarounds for broken certificates.
           trusted-first
                 Try building a chain using issuers in the trusted store first
                 to avoid problems with server-sent legacy intermediate cer-
                 tificates.  Newer versions of OpenSSL support alternative
                 chain checking and enable it by default, resulting in the
                 same behaviour; also see no-alt-chains.
     tls-ca-no-defaults-USER@HOST, tls-ca-no-defaults-HOST, tls-ca-no-defaults
           (Boolean)[Option] Do not load the default CA locations that are
           built into the used to TLS library to verify TLS server certifi-
           cates.
     tls-config-file
           [Option] If this variable is set CONF_modules_load_file(3) (if
           announced via `,+modules-load-file,' in tls-features) is used to
           allow resource file based configuration of the TLS library.  This
           happens once the library is used first, which may also be early
           during startup (logged with verbose)!  If a non-empty value is
           given then the given file, after performing Filename
           transformations, will be used instead of the TLS libraries global
           default, and it is an error if the file cannot be loaded.  The
           application name will always be passed as `s-nail'.  Some TLS
           libraries support application-specific configuration via resource
           files loaded like this, please see tls-config-module.
     tls-config-module-USER@HOST, tls-config-module-HOST, tls-config-module
           [Option] If file based application-specific configuration via
           tls-config-file is available, announced as `,+ctx-config,' by
           tls-features, indicating availability of SSL_CTX_config(3), then,
           it becomes possible to use a central TLS configuration file for all
           programs, including s-nail, for example
                 # Register a configuration section for s-nail
                 s-nail = mailx_master
                 # The top configuration section creates a relation
                 # in between dynamic SSL configuration and an actual
                 # program specific configuration section
                 [mailx_master]
                 ssl_conf = mailx_tls_config
                 # And that program specific configuration section now
                 # can map diverse tls-config-module names to sections,
                 # as in: tls-config-module=account_xy
                 [mailx_tls_config]
                 account_xy = mailx_account_xy
                 account_yz = mailx_account_yz
                 [mailx_account_xy]
                 MinProtocol = TLSv1.2
                 Curves=P-521
                 [mailx_account_yz]
                 CipherString = TLSv1.2:!aNULL:!eNULL:
                 MinProtocol = TLSv1.1
                 Options = Bugs
     tls-config-pairs-USER@HOST, tls-config-pairs-HOST, tls-config-pairs
           [Option] The value of this variable chain will be interpreted as a
           comma-separated list of directive/value pairs.  Directives and val-
           ues need to be separated by equals signs `=', any whitespace sur-
           rounding pair members is removed.  Keys are (usually) case-insensi-
           tive.  Different to when placing these pairs in a tls-config-module
           section of a tls-config-file, commas `,' need to be escaped with a
           reverse solidus `\' when included in pairs; also different: if the
           equals sign `=' is preceded with an asterisk `*' Filename
           transformations will be performed on the value; it is an error if
           these fail.  Unless proper support is announced by tls-features
           (`,+conf-ctx,') only the keys below are supported, otherwise the
           pairs will be used directly as arguments to the function
           SSL_CONF_cmd(3).
           Certificate   Filename of a TLS client certificate (chain) required
                         by some servers.  Fallback support via
                         SSL_CTX_use_certificate_chain_file(3).  Filename
                         transformations are performed.  PrivateKey will be
                         set to the same value if not initialized explicitly.
                         Some services support so-called `external' authenti-
                         cation if a TLS client certificate was successfully
                         presented during connection establishment
                         (``connecting is authenticating'').
           CipherString  A list of ciphers for TLS connections, see
                         ciphers(1).  By default no list of ciphers is set,
                         resulting in a Protocol-specific list of ciphers (the
                         protocol standards define lists of acceptable
                         ciphers; possibly cramped by the used TLS library).
                         Fallback support via SSL_CTX_set_cipher_list(3).
           Ciphersuites  A list of ciphers used for TLSv1.3 connections, see
                         ciphers(1).  These will be joined onto the list of
                         ciphers from CipherString.  Available if tls-features
                         announces `,+ctx-set-ciphersuites,', as necessary via
                         SSL_CTX_set_ciphersuites(3).
           Curves        A list of supported elliptic curves, if applicable.
                         By default no curves are set.  Fallback support via
                         SSL_CTX_set1_curves_list(3), if available.
           MaxProtocol, MinProtocol
                         The maximum and minimum supported TLS versions,
                         respectively.  Available if tls-features announces
                         `,+ctx-set-maxmin-proto,', as necessary via
                         SSL_CTX_set_max_proto_version(3) and
                         SSL_CTX_set_min_proto_version(3); these fallbacks use
                         an internal parser which understands the strings
                         `SSLv3', `TLSv1', `TLSv1.1', `TLSv1.2', `TLSv1.3',
                         and the special value `None', which disables the
                         given limit.
           Options       Various flags to set.  Fallback via
                         SSL_CTX_set_options(3), in which case any other value
                         but (exactly) `Bugs' results in an error.
           PrivateKey    Filename of the private key in PEM format of a TLS
                         client certificate.  If unset, the value of
                         Certificate is used.  Filename transformations are
                         performed.  Fallback via
                         SSL_CTX_use_PrivateKey_file(3).
           Protocol      The used TLS protocol.  If tls-features announces
                         `,+conf-ctx,' or `ctx-set-maxmin-proto' then using
                         MaxProtocol and MinProtocol is preferable.  Fallback
                         is SSL_CTX_set_options(3), driven via an internal
                         parser which understands the strings `SSLv3',
                         `TLSv1', `TLSv1.1', `TLSv1.2', `TLSv1.3', and the
                         special value `ALL'.  Multiple protocols may be given
                         as a comma-separated list, any whitespace is ignored,
                         an optional plus sign `+' prefix enables, a hyphen-
                         minus `-' prefix disables a protocol, so that `-ALL,
                         TLSv1.2' enables only the TLSv1.2 protocol.
     tls-crl-dir, tls-crl-file
           [Option] Specify a directory / a file, respectively, that contains
           a CRL in PEM format to use when verifying TLS server certificates.
     tls-features
           [Option](Read-only) This expands to a comma-separated list of the
           TLS library identity and optional features.  To ease substring
           matching the string starts and ends with a comma.  Currently sup-
           ported identities are `libressl' (LibreSSL) , `libssl-0x30000'
           (OpenSSL v3.0.0 series), `libssl-0x10100' (OpenSSL v1.1.x series)
           and `libssl-0x10000' (elder OpenSSL series, other clones).
           Optional features are preceded with a plus sign `+' when available,
           and with a hyphen-minus `-' otherwise.
           Currently known features are `conf-ctx' (tls-config-pairs),
           `ctx-config' (tls-config-module), `ctx-set-ciphersuites'
           (Ciphersuites slot of tls-config-pairs), `ctx-set-maxmin-proto'
           (tls-config-pairs), `modules-load-file' (tls-config-file), and
           `tls-rand-file' (tls-rand-file).
     tls-fingerprint-USER@HOST, tls-fingerprint-HOST, tls-fingerprint
           [Option] It is possible to replace the verification of the connec-
           tion peer certificate against the entire local pool of CAs (for
           more see Encrypted network communication) with the comparison
           against a precalculated certificate message digest, the so-called
           fingerprint, to be specified as the used tls-fingerprint-digest.
           This fingerprint can for example be calculated with `tls
           fingerprint HOST'.
     tls-fingerprint-digest-USER@HOST, tls-fingerprint-digest-HOST,
           tls-fingerprint-digest
           [Option] The message digest to be used when creating TLS certifi-
           cate fingerprints, the defaults, if available, in test order, being
           `BLAKE2s256', `SHA256'.  For the complete list of digest algorithms
           refer to smime-sign-digest.
     tls-rand-file
           [Option] If tls-features announces `,+tls-rand-file,' then this
           will be queried to find a file with random entropy data which can
           be used to seed the P(seudo)R(andom)N(umber)G(enerator), see
           RAND_load_file(3).  The default filename (RAND_file_name(3), nor-
           mally ~/.rnd) will be used if this variable is not set or empty, or
           if the Filename transformations fail.  Shall seeding the PRNG have
           been successful, RAND_write_file(3) will be called to update the
           entropy.  Remarks: libraries which do not announce this feature
           seed the PRNG by other means.
     tls-verify-USER@HOST, tls-verify-HOST, tls-verify
           [Option] Variable chain that sets the action to be performed if an
           error occurs during TLS server certificate validation against the
           specified or default trust stores tls-ca-dir, tls-ca-file, or the
           TLS library built-in defaults (unless usage disallowed via
           tls-ca-no-defaults), and as fine-tuned via tls-ca-flags.  Valid
           (case-insensitive) values are `strict' (fail and close connection
           immediately), `ask' (ask whether to continue on standard input),
           `warn' (show a warning and continue), `ignore' (do not perform val-
           idation).  The default is `ask'.
     toplines
           If defined, gives the number of lines of a message to be displayed
           with the command top; if unset, the first five lines are printed,
           if set to 0 the variable screen is inspected.  If the value is neg-
           ative then its absolute value will be used for unsigned right
           shifting (see vexpr) the screen height.
     topsqueeze
           (Boolean) If set then the top command series will strip adjacent
           empty lines and quotations.
     ttycharset
           The character set of the terminal S-nail operates on, and the one
           and only supported character set that S-nail can use if no charac-
           ter set conversion capabilities have been compiled into it, in
           which case it defaults to ISO-8859-1.  Otherwise it defaults to
           UTF-8.  Sufficient locale support provided the default will be
           preferably deduced from the locale environment if that is set (for
           example LC_CTYPE, see there for more); runtime locale changes will
           be reflected by ttycharset except during the program startup phase
           and if -S had been used to freeze the given value.  Refer to the
           section Character sets for the complete picture about character
           sets.
     typescript-mode
           (Boolean) A special multiplex variable that disables all variables
           and settings which result in behaviour that interferes with running
           S-nail in script(1); it sets colour-disable, line-editor-disable
           and (before startup completed only) termcap-disable.  Unsetting it
           does not restore the former state of the covered settings.
     umask
           For a safe-by-default policy the process file mode creation mask
           umask(2) will be set to `0077' on program startup after the
           resource files have been loaded, and unless this variable is set.
           By assigning this an empty value the active setting will not be
           changed, otherwise the given value will be made the new file mode
           creation mask.  Child processes inherit the file mode creation mask
           of their parent.
     user-HOST, user
           [v15-compat] Variable chain that sets a global fallback user name,
           used in case none has been given in the protocol and account-spe-
           cific URL.  This variable defaults to the name of the user who runs
           S-nail.
     v15-compat
           Enable upward compatibility with S-nail version 15.0 in respect to
           which configuration options are available and how they are handled.
           If set to a non-empty value the command modifier wysh is implied
           and thus enforces Shell-style argument quoting over Old-style
           argument quoting for all commands which support both.  This manual
           uses [v15-compat] and [no v15-compat] to refer to the new and the
           old way of doing things, respectively.
     verbose
           Verbose mode enables logging of informational context messages.
           Historically a (Boolean) variable, this can either be set multiple
           times (what the command line option -v uses), or be assigned a
           numeric value in order to increase verbosity.  Assigning the value
           0 disables verbosity and thus (almost) equals unset.  The maximum
           number is 3.  Also see debug.
     version, version-date, version-hexnum, version-major, version-minor,
           version-update
           (Read-only) S-nail version information: the first variable is a
           string with the complete version identification, the second the
           release date in ISO 8601 notation without time.  The third is a
           32-bit hexadecimal number with the upper 8 bits storing the major,
           followed by the minor and update version numbers which occupy 12
           bits each.  The latter three variables contain only decimal digits:
           the major, minor and update version numbers.  The output of the
           command version will include this information.
     writebackedited
           If this variable is set messages modified using the edit or visual
           commands are written back to the current folder when it is quit; it
           is only honoured for writable folders in MBOX format, though.  Note
           that the editor will be pointed to the raw message content in that
           case, i.e., neither MIME decoding nor decryption will have been
           performed, and proper mbox-rfc4155 `From_' quoting of newly added
           or edited content is also left as an exercise to the user.
ENVIRONMENT
     The term ``environment variable'' should be considered an indication that
     these variables are either standardized as vivid parts of process envi-
     ronments, or that they are commonly found in there.  The process environ-
     ment is inherited from the sh(1) once S-nail is started, and unless oth-
     erwise explicitly noted handling of the following variables transparently
     integrates into that of the INTERNAL VARIABLES from S-nail's point of
     view.  This means they can be managed via set and unset, causing auto-
     matic program environment updates (to be inherited by newly created child
     processes).
     In order to integrate other environment variables equally they need to be
     imported (linked) with the command environ.  This command can also be
     used to set and unset non-integrated environment variables from scratch,
     sufficient system support provided.  The following example, applicable to
     a POSIX shell, sets the COLUMNS environment variable for S-nail only, and
     beforehand exports the EDITOR in order to affect any further processing
     in the running shell:
           $ EDITOR="vim -u ${HOME}/.vimrc"
           $ export EDITOR
           $ COLUMNS=80 s-nail -R
     COLUMNS
           The user's preferred width in column positions for the terminal
           screen.  Queried and used once on program startup in interactive or
           batch (-#) mode, actively managed for child processes and the MLE
           (see On terminal control and line editor) in interactive mode
           thereafter.  Non-interactive mode always uses, and the fallback
           default is a compile-time constant, by default 80 columns.  If in
           batch mode COLUMNS and LINES are both set but not both are usable
           (empty, not a number, or 0) at program startup, then the real ter-
           minal screen size will be (tried to be) determined once.  (Normally
           the sh(1) manages these variables, and unsets them for pipe speci-
           fications etc.)
     DEAD  The name of the (mailbox) folder to use for saving aborted messages
           if save is set; this defaults to ~/dead.letter.  If the variable
           debug is set no output will be generated, otherwise the contents of
           the file will be replaced.  Except shell globs Filename
           transformations (also see folder) will be performed.
     EDITOR
           Pathname of the text editor to use for the edit command and ~e
           (see COMMAND ESCAPES); VISUAL is used for a more display oriented
           editor.
     HOME  The user's home directory.  This variable is only used when it
           resides in the process environment.  The calling user's home direc-
           tory will be used instead if this directory does not exist, is not
           accessible or cannot be read; it will always be used for the root
           user.  (No test for being writable is performed to allow usage by
           non-privileged users within read-only jails, but dependent on set-
           tings this directory is a default write target for, for example,
           DEAD, MBOX and more.)
     LC_ALL, LC_CTYPE, LANG
           [Option] The (names in lookup order of the) locale(7) (and / or see
           setlocale(3)) which indicates the used Character sets.  Runtime
           changes trigger automatic updates of the entire locale system,
           which includes updating ttycharset (except during startup if the
           variable has been frozen via -S).
     LINES
           The user's preferred number of lines for the terminal screen.  The
           behaviour is as described for COLUMNS, yet the compile-time con-
           stant used in non-interactive mode and as a fallback defaults to 24
           (lines).
     LISTER
           Pathname of the directory lister to use in the folders command when
           operating on local mailboxes.  Default is ls(1) (path search
           through SHELL).
     LOGNAME
           Upon startup S-nail will actively ensure that this variable refers
           to the name of the user who runs S-nail, in order to be able to
           pass a verified name to any newly created child process.
     MAIL  Is used as the user's primary system mailbox unless inbox is set.
           If the environmental fallback is also not set, a built-in compile-
           time default is used.  This is assumed to be an absolute pathname.
     MAILCAPS
           [Option] Override the default path search of The Mailcap files: any
           existing file therein will be loaded in sequence, appending any
           content to the list of MIME type handler directives.  The RFC 1524
           standard imposed default value is assigned otherwise: `~/.mailcap:
           /etc/mailcap:/usr/etc/mailcap:/usr/local/etc/mailcap'.  (The
           default value is a compile-time [Option].)
     MAILRC
           Is used as a startup file instead of ~/.mailrc if set.  In order to
           avoid side-effects from configuration files scripts should either
           set this variable to /dev/null or the -: command line option should
           be used.
     MAILX_NO_SYSTEM_RC
           If this variable is set then reading of s-nail.rc (aka
           system-mailrc) at startup is inhibited, i.e., the same effect is
           achieved as if S-nail had been started up with the option -: (and
           according argument) or -n.  This variable is only used when it
           resides in the process environment.
     MBOX  The name of the user's secondary mailbox file.  A logical subset of
           the special Filename transformations (also see folder) are sup-
           ported.  The default is ~/mbox.  Traditionally this MBOX is used as
           the file to save messages from the primary system mailbox that have
           been read.  Also see Message states.
     NETRC
           [v15-compat][Option] This variable overrides the default location
           of the user's ~/.netrc file.
     PAGER
           Pathname of the program to use for backing the command more, and
           when the crt variable enforces usage of a pager for output.  The
           default paginator is more(1) (path search through SHELL).
           S-nail inspects the contents of this variable: if its contains the
           string ``less'' then a non-existing environment variable LESS will
           be set to (the portable) `RI', likewise for ``lv'' LV will option-
           ally be set to `-c'.  Also see colour-pager.
     PATH  A colon-separated list of directories that is searched by the shell
           when looking for commands, for example
           `/bin:/usr/bin:/usr/local/bin'.
     POSIXLY_CORRECT
           This environment entry is automatically squared with posix.
     SHELL
           The shell to use for the commands !, shell, the ~! COMMAND ESCAPES
           and when starting subprocesses.  A default shell is used if this
           environment variable is not defined.
     SOCKS5_PROXY
           This environment entry is automatically squared with socks-proxy.
     SOURCE_DATE_EPOCH
           Specifies a time in seconds since the Unix epoch (1970-01-01) to be
           used in place of the current time.  This variable is looked up upon
           program startup, and its existence will switch S-nail to a repro-
           ducible mode (https://reproducible-builds.org) which uses determin-
           istic random numbers, a special fixated pseudo LOGNAME and more.
           This operation mode is used for development and by software pack-
           agers.  [v15 behaviour may differ] Currently an invalid setting is
           only ignored, rather than causing a program abortion.
                 $ SOURCE_DATE_EPOCH=`date +%s` s-nail
     TERM  [Option] The terminal type for which output is to be prepared.  For
           extended colour and font control please refer to Coloured display,
           and for terminal management in general to On terminal control and
           line editor.
     TMPDIR
           Except for the root user this variable defines the directory for
           temporary files to be used instead of /tmp (or the given compile-
           time constant) if set, existent, accessible as well as read- and
           writable.  This variable is only used when it resides in the
           process environment, but S-nail will ensure at startup that this
           environment variable is updated to contain a usable temporary
           directory.
     USER  Identical to LOGNAME (see there), but this variable is not stan-
           dardized, should therefore not be used, and is only corrected if
           already set.
     VISUAL
           Pathname of the text editor to use for the visual command and ~v
           (see COMMAND ESCAPES); EDITOR is used for a less display oriented
           editor.
FILES
     ~/.mailcap, /etc/mailcap
           [Option] Personal and system-wide MIME type handler definition
           files, see The Mailcap files.  (The shown names are part of the RFC
           1524 standard search path MAILCAPS.)
     ~/.mailrc, s-nail.rc
           User-specific and system-wide files giving initial commands, the
           Resource files.  (The used filenames come from MAILRC and
           system-mailrc, respectively.)
     ~/mbox
           The default value for MBOX.
     ~/.mime.types, /etc/mime.types
           Personal and system-wide MIME types, see The mime.types files.
     ~/.netrc
           [v15-compat][Option] The default location of the user's .netrc file
           - the section The .netrc file documents the file format.  The used
           path can be set via NETRC.
     /dev/null
           The data sink null(4).
     ~/.rnd
           [Option] Possible location for persistent random entropy seed stor-
           age, see tls-rand-file.
   Resource files
     Upon startup S-nail reads in several resource files, in order:
     s-nail.rc
           System wide initialization file (system-mailrc).  Reading of this
           file can be suppressed, either by using the -: (and according argu-
           ment) or -n command line options, or by setting the ENVIRONMENT
           variable MAILX_NO_SYSTEM_RC.
     ~/.mailrc
           File giving initial commands.  A different file can be chosen by
           setting the ENVIRONMENT variable MAILRC.  Reading of this file can
           be suppressed with the -: command line option.
     mailx-extra-rc
           Defines a startup file to be read after all other resource files.
           It can be used to specify settings that are not understood by other
           mailx(1) implementations, for example.
     The content of these files is interpreted as follows:
     o   The whitespace characters space, tabulator and newline, as well as
         those defined by the variable ifs, are removed from the beginning and
         end of input lines.
     o   Empty lines are ignored.
     o   Any other line is interpreted as a command.  It may be spread over
         multiple input lines if the newline character is ``escaped'' by plac-
         ing a reverse solidus character `\' as the last character of the
         line; whereas any leading whitespace of follow lines is ignored,
         trailing whitespace before a escaped newline remains in the input.
     o   If the line (content) starts with the number sign `#' then it is a
         comment-command and also ignored.  (The comment-command is a real
         command, which does nothing, and therefore the usual follow lines
         mechanism applies!)
     Errors while loading these files are subject to the settings of errexit
     and posix.  More files with syntactically equal content can be sourceed.
     The following, saved in a file, would be an examplary content:
            # This line is a comment command.  And y\
               es, it is really continued here.
           set debug \
               verbose
               set editheaders
   The mime.types files
     As stated in HTML mail and MIME attachments S-nail needs to learn about
     MIME (Multipurpose Internet Mail Extensions) media types in order to
     classify message and attachment content.  One source for them are
     mime.types files, the loading of which can be controlled by setting the
     variable mimetypes-load-control.  Another is the command mimetype, which
     also offers access to S-nails MIME type cache.  mime.types files have the
     following syntax:
           type/subtype extension [extension ...]
           # For example text/html html htm
     where `type/subtype' define the MIME media type, as standardized in RFC
     2046: `type' is used to declare the general type of data, while the
     `subtype' specifies a specific format for that type of data.  One or mul-
     tiple filename `extension's, separated by whitespace, can be bound to the
     media type format.  Comments may be introduced anywhere on a line with a
     number sign `#', causing the remaining line to be discarded.  S-nail also
     supports an extended, non-portable syntax in especially crafted files,
     which can be loaded via the alternative value syntax of
     mimetypes-load-control, and prepends an optional `type-marker':
           [type-marker ]type/subtype extension [extension ...]
     The following type markers are supported:
     ?     Treat message parts with this content as plain text.
     ?t    The same as plain ?.
     ?h    Treat message parts with this content as HTML tagsoup.  If the
           [Option]al HTML-tagsoup-to-text converter is not available treat
           the content as plain text instead.
     ?H    Likewise ?h, but instead of falling back to plain text require an
           explicit content handler to be defined.
     ?q    If no handler can be found a text message is displayed which says
           so.  This can be annoying, for example signatures serve a contex-
           tual purpose, their content is of no use by itself.  This marker
           will avoid displaying the text message.
     Further reading: for sending messages: mimetype,
     mime-allow-text-controls, mimetypes-load-control.  For reading etc. mes-
     sages: HTML mail and MIME attachments, The Mailcap files, mimetype,
     mime-counter-evidence, mimetypes-load-control, pipe-TYPE/SUBTYPE,
     pipe-EXTENSION.
   The Mailcap files
     [Option] RFC 1524 defines a ``User Agent Configuration Mechanism'' to be
     used to inform mail user agent programs about the locally installed
     facilities for handling various data formats, i.e., about commands and
     how they can be used to display, edit et cetera MIME part contents, as
     well as a default path search that includes multiple possible locations
     of resource files, and the MAILCAPS environment variable to overwrite
     that.  Handlers found from doing the path search will be cached, the com-
     mand mailcap operates on that cache, and the variable mailcap-disable
     will suppress automatic loading, and usage of any mailcap handlers.  HTML
     mail and MIME attachments gives a general overview of how MIME types are
     handled.
     ``Mailcap'' files consist of a set of newline separated entries.  Comment
     lines start with a number sign `#' (in the first column!) and are
     ignored.  Empty lines are ignored.  All other lines are interpreted as
     mailcap entries.  An entry definition may be split over multiple lines by
     placing the reverse solidus character `\' last in all but the final line.
     The standard does not specify how leading whitespace of successive lines
     is to be treated, therefore they are retained.
     ``Mailcap'' entries consist of a number of semicolon `;' separated
     fields.  The first two fields are mandatory and must occur in the speci-
     fied order, the remaining fields are optional and may appear in any
     order.  Leading and trailing whitespace of field content is ignored
     (removed).  The reverse solidus `\' character can be used to escape any
     following character including semicolon and itself in the content of the
     second field, and in value parts of any optional key/value field.
     The first field defines the MIME `TYPE/SUBTYPE' the entry is about to
     handle (case-insensitively).  If the subtype is specified as an asterisk
     `*' the entry is meant to match all subtypes of the named type, e.g.,
     `audio/*' would match any audio type.  The second field is the view shell
     command used to display MIME parts of the given type.
     Data consuming shell commands will be fed message (MIME part) data on
     standard input unless one or more instances of the (unquoted) string `%s'
     are used: these formats will be replaced with a temporary file(name) that
     has been prefilled with the parts data.  Data producing shell commands
     are expected to generata data on their standard output unless that format
     is used.  In all cases any given `%s' format is replaced with a properly
     shell quoted filename.  When a command requests a temporary file via `%s'
     then that will be removed again, as if the x-mailx-tmpfile and
     x-mailx-tmpfile-fill flags had been set; unless the command requests
     x-mailx-async the x-mailx-tmpfile-unlink flag is also implied; see below
     for more.
     Optional fields define single-word flags (case-insensitive), or key /
     value pairs consisting of a case-insensitive keyword, an equals sign `=',
     and a shell command; whitespace surrounding the equals sign is removed.
     Optional fields include the following:
     compose
           A program that can be used to compose a new body or body part in
           the given format.  (Currently unused.)
     composetyped
           Similar to the compose field, but is to be used when the composing
           program needs to specify the `Content-type:' header field to be
           applied to the composed data.  (Currently unused.)
     copiousoutput
           A flag field which indicates that the output of the view command is
           integrable into S-nails normal visual display.  It is mutually
           exclusive with needsterminal.
     description
           A textual description that describes this type of data.  The text
           may optionally be enclosed within double quotation marks `"'.
     edit  A program that can be used to edit a body or body part in the given
           format.  (Currently unused.)
     nametemplate
           This field specifies a filename format for the `%s' format used in
           the shell command fields, in which `%s' will be replaced by a ran-
           dom string.  (The filename is also stored in and passed to subpro-
           cesses via MAILX_FILENAME_TEMPORARY.)  The standard says this is
           ``only expected to be relevant in environments where filename
           extensions are meaningful'', and so this field is ignored unless
           the `%s' is a prefix, optionally followed by (ASCII) alphabetic and
           numeric characters, the underscore and the period.  For example, to
           specify that a JPG file is to be passed to an image viewer with a
           name ending in `.jpg', `nametemplate=%s.jpg' can be used.
     needsterminal
           This flag field indicates that the given shell command must be run
           on an interactive terminal.  S-nail will temporarily release the
           terminal to the given command in interactive mode, in non-interac-
           tive mode this entry will be entirely ignored; this flag implies
           x-mailx-noquote.
     print
           A program that can be used to print a message or body part in the
           given format.  (Currently unused.)
     test  Specifies a program to be run to test some condition, for example,
           the machine architecture, or the window system in use, to determine
           whether or not this mailcap entry applies.  If the test fails, a
           subsequent mailcap entry should be sought; also see
           x-mailx-test-once.  Standard I/O of the test program is redirected
           from and to /dev/null, and the format `%s' is not supported (the
           data does not yet exist).
     textualnewlines
           A flag field which indicates that this type of data is line-ori-
           ented and that, if encoded in `base64', all newlines should be con-
           verted to canonical form (CRLF) before encoding, and will be in
           that form after decoding.  (Currently unused.)
     x11-bitmap
           Names a file, in X11 bitmap (xbm) format, which points to an appro-
           priate icon to be used to visually denote the presence of this kind
           of data.  This field is not used by S-nail.
     x-mailx-async
           Extension flag field that denotes that the given view command shall
           be executed asynchronously, without blocking S-nail.  Cannot be
           used in conjunction with needsterminal; the standard output of the
           command will go to /dev/null.
     x-mailx-noquote
           An extension flag field that indicates that even a copiousoutput
           view command shall not be used when quoteing messages, as it would
           by default.
     x-mailx-test-once
           Extension flag which denotes whether the given test command shall
           be evaluated once only with its exit status being cached.  This is
           handy if some global unchanging condition is to be queried, like
           ``running under the X Window System''.
     x-mailx-tmpfile
           Extension flag field that requests creation of a zero-sized tempo-
           rary file, the name of which is to be placed in the environment
           variable MAILX_FILENAME_TEMPORARY.  It is an error to use this flag
           with commands that include a `%s' format (because that is imple-
           mented by means of this temporary file).
     x-mailx-tmpfile-fill
           Normally the MIME part content is passed to the handler via stan-
           dard input; if this flag is set then the data will instead be writ-
           ten into the implied x-mailx-tmpfile.  In order to cause deletion
           of the temporary file you will have to set x-mailx-tmpfile-unlink
           explicitly!  It is an error to use this flag with commands that
           include a `%s' format.
     x-mailx-tmpfile-unlink
           Extension flag field that requests that the temporary file shall be
           deleted automatically when the command loop is entered again at
           latest.  It is an error to use this flag with commands that include
           a `%s' format, or in conjunction with x-mailx-async.
           x-mailx-tmpfile is implied.
     x-mailx-last-resort
           An extension flag that indicates that this handler shall only be
           used as a last resort, when no other source (see HTML mail and MIME
           attachments) provides a MIME handler.
     x-mailx-ignore
           An extension that enforces that this handler is not used at all.
     The standard includes the possibility to define any number of additional
     fields, prefixed by `x-'.  Flag fields apply to the entire ``Mailcap''
     entry -- in some unusual cases, this may not be desirable, but differen-
     tiation can be accomplished via separate entries, taking advantage of the
     fact that subsequent entries are searched if an earlier one does not pro-
     vide enough information.  For example, if a view command needs to specify
     the needsterminal flag, but the compose command shall not, the following
     will help out the latter:
           application/postscript; ps-to-terminal %s; needsterminal
           application/postscript; ps-to-terminal %s; compose=idraw %s
     In value parts of command fields any occurrence of the format string `%t'
     will be replaced by the `TYPE/SUBTYPE' specification.  Any named parame-
     ter from a messages' `Content-type:' field may be embedded into the com-
     mand line using the format `%{' followed by the parameter name and a
     closing brace `}' character.  The entire parameter should appear as a
     single command line argument, regardless of embedded spaces, shell quot-
     ing will be performed by the RFC 1524 processor, thus:
           # Message
           Content-type:  multipart/mixed; boundary=42
           # Mailcap file
           multipart/*; /usr/local/bin/showmulti \
             %t %{boundary}  ;  composetyped  = /usr/local/bin/makemulti
           # Executed shell command
           /usr/local/bin/showmulti multipart/mixed 42
     Note that S-nail does not support handlers for multipart MIME parts as
     shown in this example (as of today).  It does not support the additional
     formats `%n' and `%F'.  An example file, also showing how to properly
     deal with the expansion of `%s', which includes any quotes that are nec-
     essary to make it a valid shell argument by itself and thus will cause
     undesired behaviour when placed in additional user-provided quotes:
           # Comment line
           text/richtext; richtext %s; copiousoutput
           text/x-perl; perl -cWT %s; nametemplate = %s.pl
           # Exit EX_TEMPFAIL=75 on signal
           application/pdf; \
             infile=%s\; \
               trap "rm -f ${infile}" EXIT\; \
               trap "exit 75" INT QUIT TERM\; \
               mupdf "${infile}"; \
             test = [ -n "${DISPLAY}" ]; \
             nametemplate = %s.pdf; x-mailx-async
           application/pdf; pdftotext -layout - -; copiousoutput
           application/*; echo "This is \\"%t\\" but \
               is 50 \% Greek to me" \; < %s head -c 512 | cat -vet; \
             copiousoutput; x-mailx-noquote; x-mailx-last-resort
     Further reading: HTML mail and MIME attachments, The mime.types files,
     mimetype, MAILCAPS, mime-counter-evidence, pipe-TYPE/SUBTYPE,
     pipe-EXTENSION.
   The .netrc file
     User credentials for machine accounts (see On URL syntax and credential
     lookup) can be placed in the .netrc file, which will be loaded and cached
     when requested by netrc-lookup.  The default location ~/.netrc may be
     overridden by the NETRC environment variable.  As long as syntax con-
     straints are honoured the file source may be replaced with the output of
     the shell command set in netrc-pipe, to load an encrypted file, for exam-
     ple.  The cache can be managed with the command netrc.
     The file consists of space, tabulator or newline separated tokens.  This
     parser implements a superset of the original BSD syntax, but users should
     nonetheless be aware of portability glitches, shall their .netrc be
     usable across multiple programs and platforms:
     o   BSD only supports double quotation marks, for example `password "pass
         with spaces"'.
     o   BSD (only?) supports escaping of single characters via a reverse
         solidus (a space could be escaped via `\ '), in- as well as outside
         of a quoted string.  This method is assumed to be present, and will
         actively be used to quote double quotation marks `"' and reverse
         solidus `\' characters inside the login and password tokens, for
         example for display purposes.
     o   BSD does not require a final quotation mark of the last user input
         token.
     o   The original BSD (Berknet) parser also supported a format which
         allowed tokens to be separated with commas - whereas at least
         Hewlett-Packard still seems to support this syntax, this parser does
         not!
     o   As a non-portable extension some widely-used programs support shell-
         style comments: if an input line starts, after any amount of white-
         space, with a number sign `#', then the rest of the line is ignored.
     o   Whereas other programs may require that the .netrc file is accessible
         by only the user if it contains a password token for any other login
         than ``anonymous'', this parser will always require these strict per-
         missions.
     Of the following list of supported tokens this parser uses (and caches)
     machine, login and password.  An existing default entry will not be used.
     machine name
           The hostname of the entries' machine, lowercase-normalized before
           use.  Any further file content, until either end-of-file or the
           occurrence of another machine or a default first-class token is
           bound (only related) to the machine name.
           As an extension that should not be the cause of any worries this
           parser supports a single wildcard prefix for name:
                 machine *.example.com login USER password PASS
                 machine pop3.example.com login USER password PASS
                 machine smtp.example.com login USER password PASS
           which would match `xy.example.com' as well as `pop3.example.com',
           but neither `example.com' nor `local.smtp.example.com'.  In the
           example neither `pop3.example.com' nor `smtp.example.com' will be
           matched by the wildcard, since the exact matches take precedence
           (it is however faster to specify it the other way around).
     default
           This is the same as machine except that it is a fallback entry that
           is used shall none of the specified machines match; only one
           default token may be specified, and it must be the last first-class
           token.
     login name
           The user name on the remote machine.
     password string
           The user's password on the remote machine.
     account string
           Supply an additional account password.  This is merely for FTP pur-
           poses.
     macdef name
           Define a macro.  A macro is defined with the specified name; it is
           formed from all lines beginning with the next line and continuing
           until a blank line is (consecutive newline characters are) encoun-
           tered.  (Note that macdef entries cannot be utilized by multiple
           machines, too, but must be defined following the machine they are
           intended to be used with.)  If a macro named init exists, it is
           automatically run as the last step of the login process.  This is
           merely for FTP purposes.
EXAMPLES
   An example configuration
           # This example assumes v15.0 compatibility mode
           set v15-compat
           # Request strict TLL transport layer security checks
           set tls-verify=strict
           # Where are the up-to-date TLS certificates?
           # (Since we manage up-to-date ones explicitly, do not use any,
           # possibly outdated, default certificates shipped with OpenSSL)
           #set tls-ca-dir=/etc/ssl/certs
           set tls-ca-file=/etc/ssl/certs/ca-certificates.crt
           set tls-ca-no-defaults
           #set tls-ca-flags=partial-chain
           wysh set smime-ca-file="${tls-ca-file}" \
             smime-ca-no-defaults #smime-ca-flags="${tls-ca-flags}"
           # This could be outsourced to a central configuration file via
           # tls-config-file plus tls-config-module if the used library allows.
           # CipherString: explicitly define the list of ciphers, which may
           #   improve security, especially with protocols older than TLS v1.2.
           #   See ciphers(1).  Possibly best to only use tls-config-pairs-HOST
           #   (or -USER@HOST), as necessary, again..
           #   Note that TLSv1.3 uses Ciphersuites= instead, which will join
           #   with CipherString (if protocols older than v1.3 are allowed)
           # Curves: especially with TLSv1.3 curves selection may be desired.
           # MinProtocol,MaxProtocol: do not use protocols older than TLS v1.2.
           #   Change this only when the remote server does not support it:
           #   maybe use chain support via tls-config-pairs-HOST / -USER@HOST
           #   to define such explicit exceptions, then, e.g.,
           #     MinProtocol=TLSv1.1
           if "$tls-features" =% ,+ctx-set-maxmin-proto,
             wysh set tls-config-pairs='\
                 CipherString=TLSv1.2:!aNULL:!eNULL:@STRENGTH,\
                 Curves=P-521:P-384:P-256,\
                 MinProtocol=TLSv1.1'
           else
             wysh set tls-config-pairs='\
                 CipherString=TLSv1.2:!aNULL:!eNULL:@STRENGTH,\
                 Curves=P-521:P-384:P-256,\
                 Protocol=-ALL\,+TLSv1.1 \, +TLSv1.2\, +TLSv1.3'
           endif
           # Essential setting: select allowed character sets
           set sendcharsets=utf-8,iso-8859-1
           # A very kind option: when replying to a message, first try to
           # use the same encoding that the original poster used herself!
           set reply-in-same-charset
           # When replying, do not merge From: and To: of the original message
           # into To:.  Instead old From: -> new To:, old To: -> merge Cc:.
           set recipients-in-cc
           # When sending messages, wait until the Mail-Transfer-Agent finishs.
           # Only like this you will be able to see errors reported through the
           # exit status of the MTA (including the built-in SMTP one)!
           set sendwait
           # Only use built-in MIME types, no mime.types(5) files
           set mimetypes-load-control
           # Default directory where we act in (relative to $HOME)
           set folder=mail
           # A leading "+" (often) means: under *folder*
           # *record* is used to save copies of sent messages
           set MBOX=+mbox.mbox DEAD=+dead.txt \
             record=+sent.mbox record-files record-resent
           # Make "file mymbox" and "file myrec" go to..
           shortcut mymbox %:+mbox.mbox myrec +sent.mbox
           # Not really optional, e.g., for S/MIME
           set from='Your Name <address AT exam.ple>'
           # It may be necessary to set hostname and/or smtp-hostname
           # if the "SERVER" of mta and "domain" of from do not match.
           # The `urlencode' command can be used to encode USER and PASS
           set mta=(smtps?|submissions?)://[USER[:PASS]@]SERVER[:PORT] \
             smtp-auth=login/plain... \
             smtp-use-starttls
           # Never refuse to start into interactive mode, and more
           set emptystart \
             colour-pager crt= \
             followup-to followup-to-honour=ask-yes fullnames \
             history-file=+.s-nailhist history-size=-1 history-gabby \
             mime-counter-evidence=0b1111 \
             prompt='?\$?!\$!/\$^ERRNAME[\$account#\$mailbox-display]? ' \
             reply-to-honour=ask-yes \
             umask=
           # Only include the selected header fields when typing messages
           headerpick type retain from_ date from to cc subject \
             message-id mail-followup-to reply-to
           # ...when forwarding messages
           headerpick forward retain subject date from to cc
           # ...when saving message, etc.
           #headerpick save ignore ^Original-.*$ ^X-.*$
           # Some mailing lists
           mlist '@xyz-editor\.xyz$' '@xyzf\.xyz$'
           mlsubscribe '^xfans@xfans\.xyz$'
           # Handle a few file extensions (to store MBOX databases)
           filetype bz2 'bzip2 -dc' 'bzip2 -zc' \
             gz 'gzip -dc' 'gzip -c'  xz 'xz -dc' 'xz -zc' \
             zst 'zstd -dc' 'zstd -19 -zc' \
             zst.pgp 'gpg -d | zstd -dc' 'zstd -19 -zc | gpg -e'
           # A real life example of a very huge free mail provider
           # Instead of directly placing content inside `account',
           # we `define' a macro: like that we can switch "accounts"
           # from within *on-compose-splice*, for example!
           define XooglX {
             set folder=~/spool/XooglX inbox=+syste.mbox sent=+sent
             set from='Your Name <address AT examp.ple>'
             set pop3-no-apop-pop.gmXil.com
             shortcut pop %:pop3s://pop.gmXil.com
             shortcut imap %:imaps://imap.gmXil.com
             # Or, entirely IMAP based setup
             #set folder=imaps://imap.gmail.com record="+[Gmail]/Sent Mail" \
             #   imap-cache=~/spool/cache
             set mta=smtp://USER:PASS AT smtp.com smtp-use-starttls
             # Alternatively:
             set mta=smtps://USER:PASS AT smtp.com:465
           }
           account XooglX {
             \call XooglX
           }
           # Here is a pretty large one which does not allow sending mails
           # if there is a domain name mismatch on the SMTP protocol level,
           # which would bite us if the value of from does not match, e.g.,
           # for people who have a sXXXXeforge project and want to speak
           # with the mailing list under their project account (in from),
           # still sending the message through their normal mail provider
           define XandeX {
             set folder=~/spool/XandeX inbox=+syste.mbox sent=+sent
             set from='Your Name <address AT exam.ple>'
             shortcut pop %:pop3s://pop.yaXXex.com
             shortcut imap %:imaps://imap.yaXXex.com
             set mta=smtps://USER:PASS AT smtp.com:465 \
               hostname=yaXXex.com smtp-hostname=
           }
           account XandeX {
             \call Xandex
           }
           # Create some new commands so that, e.g., `ls /tmp' will..
           commandalias lls '!ls ${LS_COLOUR_FLAG} -aFlrS'
           commandalias llS '!ls ${LS_COLOUR_FLAG} -aFlS'
           set pipe-message/external-body='?* echo $MAILX_EXTERNAL_BODY_URL'
           # We do not support gpg(1) directly yet.  But simple --clearsign'd
           # message parts can be dealt with as follows:
           define V {
             localopts yes
             wysh set pipe-text/plain=$'?*#++=?\
               < "${MAILX_FILENAME_TEMPORARY}" awk \
                   -v TMPFILE="${MAILX_FILENAME_TEMPORARY}" \'\
                 BEGIN{done=0}\
                 /^-----BEGIN PGP SIGNED MESSAGE-----/,/^$/ {\
                   if(done++ != 0)\
                     next;\
                   print "--- GPG --verify ---";\
                   system("gpg --verify " TMPFILE " 2>&1");\
                   print "--- GPG --verify ---";\
                   print "";\
                   next;\
                 }\
                 /^-----BEGIN PGP SIGNATURE-----/,\
                     /^-----END PGP SIGNATURE-----/{\
                   next;\
                 }\
                 {print}\
               \''
               print
           }
           commandalias V '\'call V
     When storing passwords in ~/.mailrc appropriate permissions should be set
     on this file with `$ chmod 0600 ~/.mailrc'.  If the [Option]al
     netrc-lookup is available user credentials can be stored in the central
     ~/.netrc file instead; e.g., here is a different version of the example
     account that sets up SMTP and POP3:
           define XandeX {
             set folder=~/spool/XandeX inbox=+syste.mbox sent=+sent
             set from='Your Name <address AT exam.ple>'
             set netrc-lookup
             # Load an encrypted ~/.netrc by uncommenting the next line
             #set netrc-pipe='gpg -qd ~/.netrc.pgp'
             set mta=smtps://smtp.yXXXXx.ru:465 \
                 smtp-hostname= hostname=yXXXXx.com
             set pop3-keepalive=240 pop3-no-apop-pop.yXXXXx.ru
             commandalias xp fi pop3s://pop.yXXXXx.ru
           }
           account XandeX {
             \call XandeX
           }
     and, in the ~/.netrc file:
           machine *.yXXXXx.ru login USER password PASS
     This configuration should now work just fine:
           $ echo text | s-nail -dvv -AXandeX -s Subject user AT exam.ple
   S/MIME step by step
     [Option] The first thing that is needed for Signed and encrypted messages
     with S/MIME is a personal certificate, and a private key.  The certifi-
     cate contains public information, in particular a name and email
     address(es), and the public key that can be used by others to encrypt
     messages for the certificate holder (the owner of the private key), and
     to verify signed messages generated with that certificate('s private
     key).  Whereas the certificate is included in each signed message, the
     private key must be kept secret.  It is used to decrypt messages that
     were previously encrypted with the public key, and to sign messages.
     For personal use it is recommended to get a S/MIME certificate from one
     of the major CAs on the Internet.  Many CAs offer such certificates for
     free.  Usually offered is a combined certificate and private key in
     PKCS#12 format which S-nail does not accept directly.  To convert it to
     PEM format, the following shell command can be used; please read on for
     how to use these PEM files.
           $ openssl pkcs12 -in cert.p12 -out certpem.pem -clcerts -nodes
           $ # Alternatively
           $ openssl pkcs12 -in cert.p12 -out cert.pem -clcerts -nokeys
           $ openssl pkcs12 -in cert.p12 -out key.pem -nocerts -nodes
     There is also https://www.CAcert.org which issues client and server cer-
     tificates to members of their community for free; their root certificate
     (https://www.cacert.org/certs/root.crt) is often not in the default set
     of trusted CA root certificates, though, which means their root certifi-
     cate has to be downloaded separately, and needs to be part of the S/MIME
     certificate validation chain by including it in smime-ca-dir or as a
     vivid member of the smime-ca-file.  But let us take a step-by-step tour
     on how to setup S/MIME with a certificate from CAcert.org despite this
     situation!
     First of all you will have to become a member of the CAcert.org commu-
     nity, simply by registrating yourself via the web interface.  Once you
     are, create and verify all email addresses you want to be able to create
     signed and encrypted messages for/with using the corresponding entries of
     the web interface.  Now ready to create S/MIME certificates, so let us
     create a new ``client certificate'', ensure to include all email
     addresses that should be covered by the certificate in the following web
     form, and also to use your name as the ``common name''.
     Create a private key and a certificate request on your local computer
     (please see the manual pages of the used commands for more in-depth
     knowledge on what the used arguments etc. do):
           $ openssl req -nodes -newkey rsa:4096 -keyout key.pem -out creq.pem
     Afterwards copy-and-paste the content of ``creq.pem'' into the certifi-
     cate-request (CSR) field of the web form on the CAcert.org website (you
     may need to unfold some ``advanced options'' to see the corresponding
     text field).  This last step will ensure that your private key (which
     never left your box) and the certificate belong together (through the
     public key that will find its way into the certificate via the certifi-
     cate-request).  You are now ready and can create your CAcert certified
     certificate.  Download and store or copy-and-paste it as ``pub.crt''.
     Yay.  In order to use your new S/MIME setup a combined private key/public
     key (certificate) file has to be created:
           $ cat key.pem pub.crt > ME AT HERE.paired
     This is the file S-nail will work with.  If you have created your private
     key with a passphrase then S-nail will ask you for it whenever a message
     is signed or decrypted, unless this operation has been automated as
     described in Signed and encrypted messages with S/MIME.  Set the follow-
     ing variables to henceforth use S/MIME (setting smime-ca-file is of
     interest for verification only):
           ? set smime-ca-file=ALL-TRUSTED-ROOT-CERTS-HERE \
               smime-sign-cert=ME AT HERE.paired \
               smime-sign-digest=SHA512 \
               smime-sign from=myname AT my.host
   Using CRLs with S/MIME or TLS
     [Option] Certification authorities (CAs) issue certificate revocation
     lists (CRLs) on a regular basis.  These lists contain the serial numbers
     of certificates that have been declared invalid after they have been
     issued.  Such usually happens because the private key for the certificate
     has been compromised, because the owner of the certificate has left the
     organization that is mentioned in the certificate, etc.  To seriously use
     S/MIME or TLS verification, an up-to-date CRL is required for each
     trusted CA.  There is otherwise no method to distinguish between valid
     and invalidated certificates.  S-nail currently offers no mechanism to
     fetch CRLs, nor to access them on the Internet, so they have to be
     retrieved by some external mechanism.
     S-nail accepts CRLs in PEM format only; CRLs in DER format must be con-
     verted, like, e.g.:
           $ openssl crl -inform DER -in crl.der -out crl.pem
     To tell S-nail about the CRLs, a directory that contains all CRL files
     (and no other files) must be created.  The smime-crl-dir or tls-crl-dir
     variables, respectively, must then be set to point to that directory.
     After that, S-nail requires a CRL to be present for each CA that is used
     to verify a certificate.
FAQ
     In general it is a good idea to turn on debug (-d) and / or verbose (-v,
     twice) if something does not work well.  Very often a diagnostic message
     can be produced that leads to the problems' solution.
   S-nail shortly hangs on startup
     This can have two reasons, one is the necessity to wait for a file lock
     and cannot be helped, the other being that S-nail calls the function
     uname(2) in order to query the nodename of the box (sometimes the real
     one is needed instead of the one represented by the internal variable
     hostname).  One may have varying success by ensuring that the real host-
     name and `localhost' have entries in /etc/hosts, or, more generally, that
     the name service is properly setup - and does hostname(1) return the
     expected value?  Does this local hostname have a domain suffix?  RFC 6762
     standardized the link-local top-level domain `.local', try again after
     adding an (additional) entry with this extension.
   I cannot login to Google mail (via OAuth)
     Since 2014 some free service providers classify programs as ``less
     secure'' unless they use a special authentication method (OAuth 2.0)
     which was not standardized for non-HTTP protocol authentication token
     query until August 2015 (RFC 7628).
     Different to Kerberos / GSSAPI, which is developed since the mid of the
     1980s, where a user can easily create a local authentication ticket for
     her- and himself with the locally installed kinit(1) program, that proto-
     col has no such local part but instead requires a world-wide-web query to
     create or fetch a token; since there is no local cache this query would
     have to be performed whenever S-nail is invoked (in interactive sessions
     situation may differ).
     S-nail does not directly support OAuth.  It, however, supports XOAUTH2 /
     OAUTHBEARER, see But, how about XOAUTH2 / OAUTHBEARER? If that is not
     used it is necessary to declare S-nail a ``less secure app'' (on the
     providers account web page) in order to read and send mail.  However, it
     also seems possible to take the following steps instead:
     1.   give the provider the number of a mobile phone,
     2.   enable ``2-Step Verification'',
     3.   create an application specific password (16 characters), and
     4.   use that special password instead of the real Google account pass-
          word in S-nail (for more on that see the section On URL syntax and
          credential lookup).
   But, how about XOAUTH2 / OAUTHBEARER?
     Following up I cannot login to Google mail (via OAuth) one OAuth-based
     authentication method is available: the OAuth 2.0 bearer token usage as
     standardized in RFC 6750 (according SASL mechanism in RFC 7628), also
     known as XOAUTH2 and OAUTHBEARER, allows fetching a temporary access
     token via the web that can locally be used as a password.  The protocol
     is simple and extendable, token updates or even password changes via a
     simple TLS secured server login would be possible in theory, but today a
     web browser and an external support tool are prerequisites for using this
     authentication method.  The token times out and must be periodically
     refreshed via the web.
     Some hurdles must be taken before being able to use this method.  Using
     GMail as an example, an application (that is a name) must be registered,
     for which credentials, a ``client ID'' and a ``client secret'', need to
     be created and saved locally (in a secure way).  These initial configura-
     tion steps can be performed at
           https://console.developers.google.com/apis/credentials.
     Thereafter a refresh token can be requested; a python program to do this
     for GMail accounts is
           https://github.com/google/gmail-oauth2-tools/raw/master/python/
           oauth2.py:
           $ python oauth2.py --user=EMAIL \
             --client-id=THE-ID --client-secret=THE-SECRET \
             --generate_oauth2_token
           To authorize token, visit this url and follow the directions:
             https://accounts.google.com/o/oauth2/auth?client_id=...
             Enter verification code: ...
             Refresh Token: ...
             Access Token: ...
             Access Token Expiration Seconds: 3600
           $ # Of which the last three are actual token responses.
           $ # Thereafter access tokens can regularly be refreshed
           $ # via the created refresh token (read on)
     The generated refresh token must also be saved locally (securely).  The
     procedure as a whole can be read at
           https://github.com/google/gmail-oauth2-tools/wiki/
           OAuth2DotPyRunThrough.
     Since periodic timers are not yet supported, keeping an access token up-
     to-date (from within S-nail) can only be performed via the hook
     on-main-loop-tick, or (for sending only) on-compose-enter (for more on
     authentication please see the section On URL syntax and credential
     lookup):
           set on-main-loop-tick=o-m-l-t on-compose-enter=o-c-e
           define o-m-l-t {
             xcall update_access_token
           }
           define o-c-e {
             xcall update_access_token
           }
           set access_token_=0
           define update_access_token {
             local set i epoch_sec epoch_nsec
             vput vexpr i epoch
             eval set $i # set epoch_sec/_nsec of vexpr epoch
             vput vexpr i + $access_token_ 2100
             if $epoch_sec -ge $i
               vput ! password python oauth2.py --user=EMAIL \
                   --client-id=THE-ID --client-secret=THE-SECRET \
                   --refresh-token=THE-REFRESH-TOKEN |\
                 sed '1b PASS;d; :PASS s/^.\{1,\}:\(.\{1,\}\)$/\1/'
               vput csop password trim "$password"
               if -n "$verbose"
                 echo password is <$password>
               endif
               set access_token_=$epoch_sec
             endif
           }
   Not "defunctional", but the editor key does not work
     Two thinkable situations: the first is a shadowed sequence; setting
     debug, or the most possible verbose mode, causes a printout of the bind
     tree after that is built; being a cache, this happens only upon startup
     or after modifying bindings.
     Or second, terminal libraries (see On terminal control and line editor,
     bind, termcap) may report different codes than the terminal really sends,
     rendering bindings dysfunctional because expected and received data do
     not match; the verbose listing of bindings will show the byte sequences
     that are expected.  (One common source of problems is that the -- possi-
     bly even non-existing -- keypad is not turned on, and the resulting lay-
     out reports the keypad control codes for the normal keyboard keys.)
     To overcome the situation use for example the program cat(1) with its
     option -v, if available, to see the byte sequences which are actually
     produced by keypresses, and use the variable termcap to make S-nail aware
     of them.  The terminal this is typed on produces some unexpected
     sequences, here for an example the shifted home key:
           ? set verbose
           ? bind*
           # 1B 5B=[ 31=1 3B=; 32=2 48=H
             bind base :kHOM z0
           ? x
           $ cat -v
           ^[[H
           $ s-nail -v -Stermcap='kHOM=\E[H'
           ? bind*
           # 1B 5B=[ 48=H
             bind base :kHOM z0
   Can S-nail git-send-email?
     Yes.  Put (at least parts of) the following in your ~/.gitconfig:
           [sendemail]
           smtpserver = /usr/bin/s-nail
           smtpserveroption = -t
           #smtpserveroption = -Sexpandaddr
           smtpserveroption = -Athe-account-you-need
           ##
           suppresscc = all
           suppressfrom = false
           assume8bitEncoding = UTF-8
           #to = /tmp/OUT
           confirm = always
           chainreplyto = true
           multiedit = false
           thread = true
           quiet = true
           annotate = true
     Newer git(1) versions (v2.33.0) added the option sendmailCmd.  Patches
     can also be send directly, for example:
           $ git format-patch -M --stdout HEAD^ |
             s-nail -A the-account-you-need -t RECEIVER
   Howto handle stale dotlock files
     folder sometimes fails to open MBOX mail databases because creation of
     dotlock files is impossible due to existing but unowned lock files.
     S-nail does not offer an option to deal with those files, because it is
     considered a site policy what counts as unowned, and what not.  The site
     policy is usually defined by administrator(s), and expressed in the con-
     figuration of a locally installed MTA (for example Postfix
     `stale_lock_time=500s').  Therefore the suggestion:
           $ </dev/null s-nail -s 'MTA: be no frog, handle lock' $LOGNAME
     By sending a mail to yourself the local MTA can use its normal queue
     mechanism to try the delivery multiple times, finally decide a lock file
     has become stale, and remove it.
IMAP CLIENT
     [Option]ally there is IMAP client support available.  This part of the
     program is obsolete and will vanish in v15 with the large MIME and I/O
     layer rewrite, because it uses old-style blocking I/O and makes excessive
     use of signal based long code jumps.  Support can hopefully be readded
     later based on a new-style I/O, with SysV signal handling.  In fact the
     IMAP support had already been removed from the codebase, but was rein-
     stantiated on user demand: in effect the IMAP code is at the level of
     S-nail v14.8.16 (with imapcodec being the sole exception), and should be
     treated with some care.
     IMAP uses the `imap://' and `imaps://' protocol prefixes, and an IMAP-
     based folder may be used.  IMAP URLs (paths) undergo inspections and pos-
     sible transformations before use (and the command imapcodec can be used
     to manually apply them to any given argument).  Hierarchy delimiters are
     normalized, a step which is configurable via the imap-delim variable
     chain, but defaults to the first seen delimiter otherwise.  S-nail sup-
     ports internationalised IMAP names, and en- and decodes the names from
     and to the ttycharset as necessary and possible.  If a mailbox name is
     expanded (see Filename transformations) to an IMAP mailbox, all names
     that begin with `+' then refer to IMAP mailboxes below the folder target
     box, while folder names prefixed by `@' refer to folders below the hier-
     archy base, so the following will list all folders below the current one
     when in an IMAP mailbox: `folders @'.
     Note: some IMAP servers do not accept the creation of mailboxes in the
     hierarchy base, but require that they are created as subfolders of
     `INBOX' - with such servers a folder name of the form
           imaps://mylogin AT imap.example/INBOX.
     should be used (the last character is the server's hierarchy delimiter).
     The following IMAP-specific commands exist:
     cache
           Only applicable to cached IMAP mailboxes; takes a message list and
           reads the specified messages into the IMAP cache.
     connect
           If operating in disconnected mode on an IMAP mailbox, switch to
           online mode and connect to the mail server while retaining the
           mailbox status.  See the description of the disconnected variable
           for more information.
     disconnect
           If operating in online mode on an IMAP mailbox, switch to discon-
           nected mode while retaining the mailbox status.  See the descrip-
           tion of the disconnected variable for more.  A list of messages may
           optionally be given as argument; the respective messages are then
           read into the cache before the connection is closed, thus `disco *'
           makes the entire mailbox available for disconnected use.
     imap  Sends command strings directly to the current IMAP server.  S-nail
           operates always in IMAP `selected state' on the current mailbox;
           commands that change this will produce undesirable results and
           should be avoided.  Useful IMAP commands are:
                 create         Takes the name of an IMAP mailbox as an argu-
                                ment and creates it.
                 getquotaroot   (RFC 2087) Takes the name of an IMAP mailbox
                                as an argument and prints the quotas that
                                apply to the mailbox.  Not all IMAP servers
                                support this command.
                 namespace      (RFC 2342) Takes no arguments and prints the
                                Personal Namespaces, the Other User's Names-
                                paces and the Shared Namespaces.  Each names-
                                pace type is printed in parentheses; if there
                                are multiple namespaces of the same type,
                                inner parentheses separate them.  For each
                                namespace a prefix and a hierarchy separator
                                is listed.  Not all IMAP servers support this
                                command.
     imapcodec
           Perform IMAP path transformations.  Supports vput (see Command
           modifiers), and manages the error number !.  The first argument
           specifies the operation: e[ncode] normalizes hierarchy delimiters
           (see imap-delim) and converts the strings from the locale
           ttycharset to the internationalized variant used by IMAP, d[ecode]
           performs the reverse operation.  Encoding will honour the (global)
           value of imap-delim.
     The following IMAP-specific internal variables exist:
     disconnected
           (Boolean) When an IMAP mailbox is selected and this variable is
           set, no connection to the server is initiated.  Instead, data is
           obtained from the local cache (see imap-cache).  Mailboxes that are
           not present in the cache and messages that have not yet entirely
           been fetched from the server are not available; to fetch all mes-
           sages in a mailbox at once, the command `copy * /dev/null' can be
           used while still in connected mode.  Changes that are made to IMAP
           mailboxes in disconnected mode are queued and committed later when
           a connection to that server is made.  This procedure is not com-
           pletely reliable since it cannot be guaranteed that the IMAP unique
           identifiers (UIDs) on the server still match the ones in the cache
           at that time.  Data is saved to DEAD when this problem occurs.
     disconnected-USER@HOST
           The specified account is handled as described for the disconnected
           variable above, but other accounts are not affected.
     imap-auth-USER@HOST, imap-auth
           Sets the IMAP authentication method.  Supported are the default
           `login', [v15-compat] `oauthbearer' (see FAQ entry But, how about
           XOAUTH2 / OAUTHBEARER?), [v15-compat] `external' and `externanon'
           (for TLS secured connections which pass a client certificate via
           tls-config-pairs), as well as the [Option]al `cram-md5' and
           `gssapi'.  All methods need a user and a password except `gssapi'
           and `external', which only need the former.  `externanon' solely
           builds upon the credentials passed via a client certificate, and is
           usually the way to go since tested servers do not actually follow
           RFC 4422, and fail if additional credentials are actually passed.
     imap-cache
           Enables caching of IMAP mailboxes.  The value of this variable must
           point to a directory that is either existent or can be created by
           S-nail.  All contents of the cache can be deleted by S-nail at any
           time; it is not safe to make assumptions about them.
     imap-delim-USER@HOST, imap-delim-HOST, imap-delim
           The hierarchy separator used by the IMAP server.  Whenever an IMAP
           path is specified it will undergo normalization.  One of the nor-
           malization steps is the squeezing and adjustment of hierarchy sepa-
           rators.  If this variable is set, any occurrence of any character
           of the given value that exists in the path will be replaced by the
           first member of the value; an empty value will cause the default to
           be used, it is `/.'.  If not set, we will reuse the first hierarchy
           separator character that is discovered in a user-given mailbox
           name.
     imap-keepalive-USER@HOST, imap-keepalive-HOST, imap-keepalive
           IMAP servers may close the connection after a period of inactivity;
           the standard requires this to be at least 30 minutes, but practical
           experience may vary.  Setting this variable to a numeric `value'
           greater than 0 causes a `NOOP' command to be sent each `value' sec-
           onds if no other operation is performed.
     imap-list-depth
           When retrieving the list of folders on an IMAP server, the folders
           command stops after it has reached a certain depth to avoid possi-
           ble infinite loops.  The value of this variable sets the maximum
           depth allowed.  The default is 2.  If the folder separator on the
           current IMAP server is a slash `/', this variable has no effect and
           the folders command does not descend to subfolders.
     imap-use-starttls-USER@HOST, imap-use-starttls-HOST, imap-use-starttls
           Causes S-nail to issue a `STARTTLS' command to make an unencrypted
           IMAP session TLS encrypted.  This functionality is not supported by
           all servers, and is not used if the session is already encrypted by
           the IMAPS method.
SEE ALSO
     bogofilter(1), gpg(1), more(1), newaliases(1), openssl(1), sendmail(1),
     sh(1), spamassassin(1), iconv(3), setlocale(3), aliases(5), termcap(5),
     terminfo(5), locale(7), mailaddr(7), re_format(7) (or regex(7)),
     mailwrapper(8), sendmail(8)
HISTORY
     M. Douglas McIlroy writes in his article ``A Research UNIX Reader:
     Annotated Excerpts from the Programmer's Manual, 1971-1986'' that a
     mail(1) command already appeared in First Edition UNIX in 1971:
           Electronic mail was there from the start.  Never satisfied with its
           exact behavior, everybody touched it at one time or another: to
           assure the safety of simultaneous access, to improve privacy, to
           survive crashes, to exploit uucp, to screen out foreign free-
           loaders, or whatever.  Not until v7 did the interface change
           (Thompson).  Later, as mail became global in its reach, Dave Pre-
           sotto took charge and brought order to communications with a grab-
           bag of external networks (v8).
     BSD Mail, in large parts compatible with UNIX mail, was written in 1978
     by Kurt Shoens and developed as part of the BSD UNIX distribution until
     1995.  This manual page is derived from ``The Mail Reference Manual''
     that Kurt Shoens wrote for Mail 1.3, included in 3BSD in 1980.  The com-
     mon UNIX and BSD denominator became standardized as mailx(1) in the
     X/Open Portability Guide Issue 2 (January 1987).  After the rise of Open
     Source BSD variants Mail saw continuous development in the individual
     code forks, noticeably by Christos Zoulas in NetBSD.  Based upon this
     Nail, later Heirloom Mailx, was developed by Gunnar Ritter in the years
     2000 until 2008.  Since 2012 S-nail is maintained by Steffen Nurpmeso.
     Electronic mail exchange in general is a concept even older.  The earli-
     est well documented electronic mail system was part of the Compatible
     Time Sharing System (CTSS) at MIT, its MAIL command had been proposed in
     a staff planning memo at the end of 1964 and was implemented in mid-1965
     when Tom Van Vleck and Noel Morris wrote the necessary code.  Similar
     communication programs were built for other timesharing systems.  One of
     the most ambitious and influential was Murray Turoff's EMISARI.  Created
     in 1971 for the United States Office of Emergency Preparedness, EMISARI
     combined private electronic messages with a chat system, public postings,
     voting, and a user directory.
     During the 1960s it was common to connect a large number of terminals to
     a single, central computer.  Connecting two computers together was rela-
     tively unusual.  This began to change with the development of the
     ARPANET, the ancestor of today's Internet.  In 1971 Ray Tomlinson adapted
     the SNDMSG program, originally developed for the University of California
     at Berkeley timesharing system, to give it the ability to transmit a mes-
     sage across the network into the mailbox of a user on a different com-
     puter.  For the first time it was necessary to specify the recipient's
     computer as well as an account name.  Tomlinson decided that the under-
     used commercial at `@' would work to separate the two.
     Sending a message across the network was originally treated as a special
     instance of transmitting a file, and so a MAIL command was included in
     RFC 385 on file transfer in 1972.  Because it was not always clear when
     or where a message had come from, RFC 561 in 1973 aimed to formalize
     electronic mail headers, including ``from'', ``date'', and ``subject''.
     In 1975 RFC 680 described fields to help with the transmission of mes-
     sages to multiple users, including ``to'', ``cc'', and ``bcc''.  In 1977
     these features and others went from best practices to a binding standard
     in RFC 733.  Queen Elizabeth II of England became the first head of state
     to send electronic mail on March 26 1976 while ceremonially opening a
     building in the British Royal Signals and Radar Establishment (RSRE) in
     Malvern.
AUTHORS
     Kurt Shoens, Edward Wang, Keith Bostic, Christos Zoulas, Gunnar Ritter.
     S-nail is developed by Steffen Nurpmeso <s-mailx AT lists.eu>.
CAVEATS
     [v15 behaviour may differ] Interrupting an operation via SIGINT aka
     `control-C' from anywhere else but a command prompt is very problematic
     and likely to leave the program in an undefined state: many library func-
     tions cannot deal with the siglongjmp(3) that this software (still) per-
     forms; even though efforts have been taken to address this, no sooner but
     in v15 it will have been worked out: interruptions have not been disabled
     in order to allow forceful breakage of hanging network connections, for
     example (all this is unrelated to ignore).
     The SMTP and POP3 protocol support of S-nail is very basic.  Also, if it
     fails to contact its upstream SMTP server, it will not make further
     attempts to transfer the message at a later time (setting save and
     sendwait may be useful).  If this is a concern, it might be better to set
     up a local SMTP server that is capable of message queuing.
BUGS
     When a network-based mailbox is open, directly changing to another net-
     work-based mailbox of a different protocol (i.e., from POP3 to IMAP or
     vice versa) will cause a ``deadlock''.
     After deleting some message of a POP3 mailbox the header summary falsely
     claims that there are no messages to display, one needs to perform a
     scroll or dot movement to restore proper state.
     In `thread'ed sort mode a power user may encounter crashes very occasion-
     ally (this is may and very).
     Please report bugs to the contact-mail address, for example from within
     s-nail: `? eval mail $contact-mail'.  Including the verbose output of the
     command version may be helpful:
           ? wysh set escape=! verbose; vput version xy; unset verbose;\
             eval mail $contact-mail
           Bug subject
           !I xy
           !.
     Information on the web at `$ s-nail -X 'echo $contact-web; x''.
BSD                            November 11, 2021                           BSD