SpamAssassin - Use this program to "Unspam" your mailbox(es).
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.
Scrollout F1 - Scrollout is a free and easy to use anti-spam email gateway designed for Linux & Windows administrators without advanced email security experience.
Oikik Antispam - Oikik is a cloud-based email filtering service that works with your existing email service.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).