dmarcian - Safeguard your email from abuse
SpamHero - SpamHero is a professional spam filtering accuracy while lowering the cost to domain and business owners.
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).
Barracuda Email Protection - Barracuda Essentials offers advanced Office 365 security features including email scanning, archiving, backup & recovery, and real-time phishing defense.
Mimecast - Utilities, Application Utilities, and Security