sendmail-wrapper: initial draft

crux-wiki/sendmail-wrapper

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+==== SSH session that runs sendmail instead of an interactive shell ====
+
+=== What? ===
+A wrapper for your local mail transfer agent (MTA) that initiates an SSH
+session when your pubnix email address appears in the "From:" header.
+
+=== Why? ===
+You want to send email from your pubnix address without running the risk
+that the receiving server will reject it due to a mismatch between the
+"From" header and your ISP's IP block, but ...
+
+  * the low volume of your outgoing mail cannot justify the cost of an
+    upgraded membership, or
+  * your Internet connection has too high a latency to be running a
+    full-screen editor within an SSH session.
+
+=== Where? ===
+a *BSD, Linux, or WSL environment with a client that generates
+MIME-formatted mail (as expected by sendmail-compatible MTAs such as
+msmtp)
+
+=== How? ===
+  - Set up [[ssh public key authentication|SSH public key authentication]],
+    if you have not already done so. This step can be omitted if you
+    prefer to type your password for each outgoing mail.
+  - Save the wrapper script below somewhere in your $PATH. Change the
+    value of $MTA to the mail transport agent that you would otherwise be
+    using for outgoing mail, the value of $PATH if any of the commands live in
+    non-standard locations, and of course all the hard-coded email addresses
+    in the script.
+  - Configure your mail user agent (MUA) to call the wrapper script,
+    rather than calling your MTA directly.
+
+<code>
+#!/usr/bin/env bash
+#
+# msmtp.wrap - bypass local in favor of remote sendmail,
+#              if certain headers are found
+
+PATH=/bin:/usr/bin:/usr/local/bin
+Account0=me@pubnix-0.org
+Account1=me@other-pubnix.net
+MTA=msmtp
+draft="$(mktemp -t sendmail.XXXXXX)"
+cat /dev/stdin > "$draft"
+sender="$(grep '^From:' "$draft" | head -n 1 | cut -d: -f2)"
+
+case "$sender" in
+    *$Account0*)
+        sendmail="ssh $Account0 sendmail -t" ;;
+    *$Account1*)
+        sendmail="ssh $Account1 sendmail -t" ;;
+    *)
+        sendmail="$MTA $*" ;;
+esac
+
+< "$draft" $sendmail; status=$?
+rm -f "$draft"; exit $status
+</code>
+
+=== What Next? ===
+
+The possibilities for customization of this wrapper are only limited by
+your imagination. You can parse the outgoing messages to look for
+particular recipients, subject lines, etc., and define a custom $sendmail
+command to handle each case. On the pubnix side, the arguments following
+''ssh $Account0'' will be passed to your usual login shell after the
+''-c'' option(( see the sshd(8) man-page, [[ https://man.openbsd.org/sshd ]] )),
+so you will enjoy all the environment variables defined in
+''$HOME/.bash_login'' if you use bash as your login shell. Save your
+custom scripts in a directory that your pubnix login shell considers part of
+$PATH, and you can then perform all kinds of tasks (like updating your
+website) without leaving your mail client.