SpamAssassin is, of course, a mail filter to identify spam. See http://SpamAssassin.org/ for more details.

This flexible and powerful set of Perl programs is so powerful because, unlike older spam filtering approaches, it uses the combined score from multiple types of checks to determine if a given message is spam.

Its primary features are:

• Header tests
• Body phrase tests
• Bayesian filtering
• Automatic address whitelist/blacklist
• Manual address whitelist/blacklist
• Collaborative spam identification databases (DCC, Pyzor, Razor2). See NetworkTests
• RBL (Realtime Blackhole Lists)
• Character sets and locales
  

Even though any one of these tests might, by themselves, misidentify a ham or spam, their combined score is terribly difficult to fool.