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.
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Geary - Geary is an email application built for GNOME 3.
mailcow - An open source mailserver suite.
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.