iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Mailtraq - Mailtraq is a commercial groupware and webmail server system for the Windows operating systems.
Zimbra - Zimbra is trusted by over 500 million users to increase productivity with a complete set of collaboration tools while maintaining total control over security and privacy.
mailcow - An open source mailserver suite.
Eudora Internet Mail Server - Eudora Internet Mail Server is a highly advanced internet mail system that supports the all kind of email systems and clients either it is IMAP, POP3, SMTP and much more.
Microsoft Exchange Server - Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft.