OBM - OBM is a free messaging and collaboration platform useful for just a couple to many thousands of users with strong support for mobile devices.
Dovecot - Dovecot is an open source IMAP and POP3 email server for Linux/UNIX-like systems, written with...
Microsoft Exchange Server - Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft.
Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
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.
Courier IMAP - The Courier IMAP server is a fast, scalable, enterprise IMAP server that uses Maildirs.