Open Source Tools That Can Replace Popular Security Software.
1) ASSP: (short for "Anti-Spam SMTP Proxy") humbly calls itself "the absolute best SPAM fighting weapon that the world has ever known!" It works with most SMTP servers to stop spam and scan for viruses (using ClamAV). Operating System: OS Independent.
2) MailScanner: Used by more than 100,000 sites, MailScanner leverages Apache's SpamAssassin project and ClamAV to provide anti-spam and anti-virus capabilities. It's designed to sit on corporate mail gateways or ISP servers to protect end users from threats. Operating System: OS Independent.
3) SpamAssassin: This Apache project declares itself "the powerful #1 open-source spam filter." It uses a variety of different techniques, including header and text analysis, Bayesian filtering, DNS blocklists, and collaborative filtering databases, to filter out bulk e-mail at the mail server level. Operating System: primarily Linux and OS X, although Windows versions are available. More