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).
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
mailcow - An open source mailserver suite.
The Bat - The Bat! is a secure desktop email client for Windows, designed to protect your correspondence against third-parties and to save your time.