Proxmox Mail Gateway - Protect mail server against spam, viruses and malware with Proxmox Mail Gateway security for businesses, government, education and non-profit organizations.
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.
SpamAssassin - Use this program to "Unspam" your mailbox(es).
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...
mailcow - An open source mailserver suite.