Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Evolution - Apps/Evolution - GNOME Wiki!
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.
Umso - The Website Builder for Startups
Mutt - Mutt is a small but very powerful text-based mail client for Unix operating systems.