SpamAssassin

Sys admins can field scores of complaints and spend months testing software suites that turn out to be too aggressive , too passive, or too complicated to setup only to discover that SpamAssassin (SA), the leading open source spam-fighting tool, is free, flexible, powerful, highly-regarded, and remarkably effective. The drawback? Until now, it was SpamAssassin's lack of published documentation. This clear, concise new guide provides the expertise you need to take back your inbox.

"Detailed, accurate and informative--recommended for spam-filtering beginners and experts alike." --Justin Mason, SpamAssassin development team