Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Mutt - Mutt is a small but very powerful text-based mail client for Unix operating systems.
Geary - Geary is an email application built for GNOME 3.
Alpine - Alpine is a fast, easy to use email client that is suitable for both the inexperienced email user...
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!