Proofpoint - Proofpoint provides cloud-based email security, e-discovery and compliance solutions for companies to protect sensitive business data.
SpamAssassin - Use this program to "Unspam" your mailbox(es).
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.
MailScanner - MailScanner is a open source email security system design for Linux-based email gateways.
dmarcian - Safeguard your email from abuse
Rspamd - Rspamd is an advanced spam filtering system that allows evaluation of messages by a number of rules...