SpamAssassin - Use this program to "Unspam" your mailbox(es).
SpamHero - SpamHero is a professional spam filtering accuracy while lowering the cost to domain and business owners.
MailScanner - MailScanner is a open source email security system design for Linux-based email gateways.
SpamTitan - Comprehensive Email Protection. 99.99% Spam Detection. Blocks phishing, viruses, malware and other email threats.
Rspamd - Rspamd is an advanced spam filtering system that allows evaluation of messages by a number of rules...
Zimbra - Zimbra is trusted by over 500 million users to increase productivity with a complete set of collaboration tools while maintaining total control over security and privacy.