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.
mailcow - An open source mailserver suite.
Geary - Geary is an email application built for GNOME 3.
Roundcube - Web-based IMAP email client
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.