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.
Exchange - On-demand technical interview coaching by engineers
Axigen - Axigen is an integrated email, calendaring & collaboration platform, built on a Linux mail server technology.