SMTP
- agorum core Open Source Document Management/Enterprise Content Management
- Atmail
- Axigen
- Bongo
- Citadel
- ContactOffice
- CommuniGate Pro
- Courier
- Eudora Internet Mail Server
- Exim
- FirstClass
- fmaild
- Gammadyne Mailer
- Gordano Messaging Suite
- Hexamail Server
- HMailServer
- IBM Lotus Domino
- IceWarp (IceWarp / Merak Mail Server)
- Ipswitch IMail Server
- James (Java Apache Mail Enterprise Server)
- Kerio Connect
- MagicMail
- Mailtraq
- MDaemon Email Server for Windows by Alt-N Technologies
- Mercury Mail Transport System
- Meldware Communication Suite
- MeTA1 (successor of the sendmail X project)
- Microsoft Exchange Server
- MMDF
- Novell GroupWise
- Novell NetMail
- OpenSMTPD
- Openwave Systems
- Open-Xchange
- Oracle Beehive
- Oracle Communications Messaging Server
- Postfix
- qmail
- qpsmtpd
- Scalix
- Sendmail
- Smail
- SparkEngine
- SMTP Proxy
- Sun Java System
- Synovel Collabsuite ( Linux mail server )
- WinGate
- XMail
- XMS Email Application Server
- Zarafa
- Zimbra
- ZMailer
Read more about this topic: List Of Mail Servers