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.
hMail Server - hMailServer is a free e-mail server for Microsoft Windows.
mailcow - An open source mailserver suite.
Roundcube - Web-based IMAP email client
Axigen - Axigen is an integrated email, calendaring & collaboration platform, built on a Linux mail server technology.