Screaming Penguin - Source Does Matter

SpamAssassin is a filter for, you guessed it, spam. SpamAssassin kicks total arse, it includes normal stuff like blacklists and so on, fancy stuff like Bayesian filtering, and its own set of tests using the content, headers and source of email. Setup on an email server as a filter its really incredible stuff.

That said a major new release is out that improves efficiency and improves the testing capabilities. Check it out SpamAssassin 2.60.   http://spamassassin.taint.org/