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.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Mailbird - Mailbird is the best email client for Windows 7, 8 and 10
Geary - Geary is an email application built for GNOME 3.
EmailTray - EmailTray is an email client and an email notifier for Gmail, Hotmail, Yahoo mail and AOL. EmailTray ranks messages and alerts you to the most important ones.