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.
Geary - Geary is an email application built for GNOME 3.
Mutt - Mutt is a small but very powerful text-based mail client for Unix operating systems.
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.
aerc - Highly efficient and extensible email client for the terminal