SpamAssassin - Use this program to "Unspam" your mailbox(es).
Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
MailScanner - MailScanner is a open source email security system design for Linux-based email gateways.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
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.