Hornetsecurity - Hornetsecurity offers you comprehensive, integrated managed e-mail security services.

Proxmox Mail Gateway - Protect mail server against spam, viruses and malware with Proxmox Mail Gateway security for businesses, government, education and non-profit organizations.

Mimecast Gateway - Mimecast Gateway is a SaaS-based email security solution that uses sophisticated, multi-layered detection engines and intelligence to protect email data and employees from malware, spam, advanced threats and zero-day attacks.

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).