Title: Notes on .forward
Author: Alexander Arkhipov <aa@manpager.org>
Created: 2024-06-25
Modified: 2024-06-25

You can put a .forward file in your home directory to instruct the
site's smtpd server what to do with your email. It contains entries in
two formats:

user@site.tld
"|/path/to/mda"

The first simply tells the smtpd to deliver your mail to user@site.tld
instead, which is already useful if you usually read your mail on
another machine, or from a different account.

The second pipes the message to a mail delivery program. An example of
such program is fdm, and you may already be familiar with its more
common uses. Here's something more interesting: suppose you want to
share your ZucBook account with Bob. Rather than manually sending the
one time code to Bob whenever he changes his IP address, and ZucBook
decides that his login is a bit suspicios, you'd probably prefer to
forward these to Bob automatically, but also keep a copy for yourself.
Here's one way to do this with fdm, mblaze and .forward.

First, write the following .fdm.conf:

account "forward" disabled stdin

match account "forward" {
	match "^from:.*<noreply@zucbook\.com>" in headers and
	    "^subject:.* verification code is " in headers actions {
		# save a copy for ourselves
		maildir "%h/mail"
		# and forward to bob with mblaze
		exec "mfwd -to bob@mail.tld -send -- %[mail_file]"
	}
	# otherwise just save for ourselves
	match all action maildir "%h/mail"
}

And then the following .forward:

"|/usr/local/bin/fdm -m -a forward fetch"