http://www.sendmail.org/ — Offers a thorough technical breakdown of Sendmail features, documentation and configuration examples.

http://www.sendmail.com/ — Contains news, interviews and articles concerning Sendmail, including an expanded view of the many options available.

http://www.postfix.org/ — The Postfix project home page contains a wealth of information about Postfix. The mailing list is a particularly good place to look for information.

http://fetchmail.berlios.de/ — The home page for Fetchmail, featuring an online manual, and a thorough FAQ.

http://www.procmail.org/ — The home page for Procmail with links to assorted mailing lists dedicated to Procmail as well as various FAQ documents.

http://partmaps.org/era/procmail/mini-faq.html — An excellent Procmail FAQ, offers troubleshooting tips, details about file locking, and the use of wildcard characters.

http://www.uwasa.fi/~ts/info/proctips.html — Contains dozens of tips that make using Procmail much easier. Includes instructions on how to test .procmailrc files and use Procmail scoring to decide if a particular action should be taken.

http://www.spamassassin.org/ — The official site of the SpamAssassin project.