Mutt
Mutt, MUTT - The Mongrel of Mail User Agents, MUTT - the mongrel of mailers, Mutt For The Ridiculous User, Mutt-users, My First Mutt, The Mutt FAQ, The Online Mutt Manual, Yahoo Groups: mutt-dev
Procmail
Spam Filtering, Tutorials, Automatic PGP Key Retrieval in Procmail, Cameron Laird's Personal Notes on Procmail, Filtering Mail FAQ, ii.com: Processing Mail with Procmail, ii.com: Procmail Quick Start, junkfilter, Mailagent ftp directory (Perl/Unix), Procmail, Procmail and PGP, Procmail FAQ
X
Arrow, Balsa, Exmh, GNUMail.app - A Mail.app clone for GNUstep., Ishmail, Mailliam, Postilion, Pronto, Ratatosk, Sandmail
archivemail
Python tool for detecting and compressing older e-mail messages.
BSD Vacation V2
RFP-compliant vacation mailer, designed to replace the standard Sendmail vacation program.
Fetchmail
Fetchmail is a free, full-featured, robust, well-documented remote-mail retrieval and forwarding utility intended to be used over on-demand TCP/IP links (such as SLIP or PPP connections).
maildrop - mail delivery agent with filtering
Maildrop reads a mail message from standard input, then delivers the message to your mailbox. maildrop will optionally read instructions from a file, which describes how to filter incoming mail. Instructions can be provided having mail delivered to alternate mailboxes, or forwarded using a structured filtering language.
Mailfilt, by Mike Fisk (Unix/Perl)
Mailfilt is a procmail-like Perl application for filtering your mail. (Last updated sometime in 1995 ...)
Pygmy
A GNOME mail client written in Python. Pygmy supports attachments and MIME messages, both in the message composer and in the message viewer.
The Fetchmail FAQ
Frequently asked questions about fetchmail.
Vacation Auto-Reply (Unix)
Simple instructions for one variant of the vacation(1) program on Unix. (This doesn't appear to be the most standard version, but the general instructions look relatively ok.)