SpamAssassin is a mature, widely-deployed open source project that serves as a mail filter to identify Spam. SpamAssassin uses a variety of mechanisms including header and text analysis, Bayesian filtering, DNS blocklists, and collaborative filtering databases. SpamAssassin runs on a server, and filters spam before it reaches your mailbox
3.1.5 includes a large number of bug fixes and documentation updates.
Downloads are available from:
http://spamassassin.apache.org/downloads.cgi?update=200608300000