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.
Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
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
Cyrus IMAP - The Cyrus IMAP server differs from other IMAP server implementations in that it is generally...