Useful Websites

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.