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.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
mailcow - An open source mailserver suite.
Roundcube - Web-based IMAP email client
Modoboa - Modoboa is a mail hosting and management platform including a modern and simplified Web User Interface.
DistroWatch - 9 hours ago - It does include a proprietary software set optimized for business users, students, researchers and developers." A list of features and further details can be found in the company's release announcement.