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).
Roundcube - Web-based IMAP email client
mailcow - An open source mailserver suite.
Cyrus IMAP - The Cyrus IMAP server differs from other IMAP server implementations in that it is generally...
Cypht - All your E-mail, from all your accounts, in one place. Cypht is like a news reader, but for E-mail.