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!
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.
Geary - Geary is an email application built for GNOME 3.
Kontact - KMail - Email client by KDE, supporting POP3 and IMAP mailboxes.
Mu4e - Starting with version 0.9.8, mu provides an emacs-based e-mail client which uses mu as its back-end: mu4e.