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.
Cyrus IMAP - The Cyrus IMAP server differs from other IMAP server implementations in that it is generally...
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Roundcube - Web-based IMAP email client
mailcow - An open source mailserver suite.
Blue-Mind - Blue Mind is a complete solution of company messaging, calendars and collaborative work.