Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Mu4e - Starting with version 0.9.8, mu provides an emacs-based e-mail client which uses mu as its back-end: mu4e.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Mutt - Mutt is a small but very powerful text-based mail client for Unix operating systems.
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.
aerc - Highly efficient and extensible email client for the terminal