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.
Mu4e - Starting with version 0.9.8, mu provides an emacs-based e-mail client which uses mu as its back-end: mu4e.
mailcow - An open source mailserver suite.
Mutt - Mutt is a small but very powerful text-based mail client for Unix operating systems.
Roundcube - Web-based IMAP email client
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.