SpamAssassin

http://www.spamassassin.org/

"SpamAssassin is a mail filter to identify spam. Using its rule base, it uses a wide range of heuristic tests on mail headers and body text to identify "spam", also known as unsolicited commercial email."

FAQ: http://wiki.apache.org/spamassassin/TheFaqHasMoved

Some notes on site wide use, per user configuration, using spamc/spamd for performance, and procmail filtering can be found at: http://ist.uwaterloo.ca/~dkeenan/talks/spamassassin/

Works with Razor, a distributed, collaborative, spam detection and filtering network: http://razor.sourceforge.net/

Also check out BogoFilter

To use SA together with Postfix, Cyrus, Sieve, and virtual domains/users, use the Python script contrib/spamcheck.py that is included with SpamAssassin. Let it update the headers and defang the message. That way each user can set up Sieve filters to look at the headers and move/trash the message as they wish.

I set up a combination of fetchmail, SpamAssassin, Razor, and fetchmail for a user (let's call him B). Previously B was (well, still is) getting 10-15 spams per day. All of it was in Korean from Korean addresses. Besides HTML spam from random Korean "businesses", there was an increasing number of animated (.gif) porn... very nasty. So far, it's been 100% accurate - all the spam is getting filtered to a caught spam folder, and good email stays in INBOX, including mailing list and promotional stuff he signed up for. Eventually, once there's a collection of spam I'll feed it through BogoFilter.

http://freshmeat.net/projects/amavisd-new/ - Amavisd-new nicely ties together SpamAssassin with various virii scanners such as ClamAV: http://www.clamav.net/

For tips on using amavisd-new with SA: http://www.ijs.si/software/amavisd/#faq-spam

http://wiki.apache.org/spamassassin/CustomRulesets - custom rules for SpamAssassin