(Via Open Source)
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.
Free software: it is distributed under the same terms and conditions as other popular open-source software packages such as the Apache web server.
Lab Test Results: Side by side testing of the Barracuda Networks Model 300 (Anti Spam / Anti Virus) appliance and Data Center West's internal cluster of Linux Anti Spam and Anti Virus servers, resulted in the internal Linux server clusters out performing the Barracuda Model 300 40 to 1.