Rspamd - Rspamd is an advanced spam filtering system that allows evaluation of messages by a number of rules...
dmarcian - Safeguard your email from abuse
SpamAssassin - Use this program to "Unspam" your mailbox(es).
Proofpoint - Proofpoint provides cloud-based email security, e-discovery and compliance solutions for companies to protect sensitive business data.
Proxmox Mail Gateway - Protect mail server against spam, viruses and malware with Proxmox Mail Gateway security for businesses, government, education and non-profit organizations.
Barracuda Email Protection - Barracuda Essentials offers advanced Office 365 security features including email scanning, archiving, backup & recovery, and real-time phishing defense.