Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
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.
Geary - Geary is an email application built for GNOME 3.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Kontact - KMail - Email client by KDE, supporting POP3 and IMAP mailboxes.
mailcow - An open source mailserver suite.