Microsoft Exchange Server - Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft.
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.
Citadel - Is a collaboration suite (messaging and groupware).
MailEnable - MailEnable is a Windows based mail server and email server program that is packed with the state of the features and functions like antivirus and spam filtering system, webmail management system, cardDAV, CalDAV and much more.
Squirrelmail - SquirrelMail is a standards-based webmail package written in PHP.
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.