Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Sendmail - Sendmail is a general purpose internetwork email routing facility that supports many kinds of...
Geary - Geary is an email application built for GNOME 3.
Exim - Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet.
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.