procmail is a powerful application used to filter incoming mail. It allows users to define “rules” which can be matched to incoming mails to perform specific functions or to reroute mail to alternative mailboxes or email addresses. procmail can be installed using the mail/procmail port. Once installed, it can be directly integrated into most MTAs. Consult the MTA documentation for more information. Alternatively, procmail can be integrated by adding the following line to a .forward in the home directory of the user:

"|exec /usr/local/bin/procmail || exit 75"

The following section displays some basic procmail rules, as well as brief descriptions of what they do. Rules must be inserted into a .procmailrc, which must reside in the user's home directory.

The majority of these rules can be found in procmailex(5).

To forward all mail from <user@example.com> to an external address of <goodmail@example2.com>:

:0
* ^From.*user@example.com
! goodmail@example2.com

To forward all mails shorter than 1000 bytes to an external address of <goodmail@example2.com>:

:0
* < 1000
! goodmail@example2.com

To send all mail sent to <alternate@example.com> to a mailbox called alternate:

:0
* ^TOalternate@example.com
alternate

To send all mail with a subject of “Spam” to /dev/null:

:0
^Subject:.*Spam
/dev/null

A useful recipe that parses incoming FreeBSD.org mailing lists and places each list in its own mailbox:

:0
* ^Sender:.owner-freebsd-\/[^@]+@FreeBSD.ORG
{
	LISTNAME=${MATCH}
	:0
	* LISTNAME??^\/[^@]+
	FreeBSD-${MATCH}
}