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!
Geary - Geary is an email application built for GNOME 3.
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.
Kontact - KMail - Email client by KDE, supporting POP3 and IMAP mailboxes.
Sylpheed - Sylpheed is a simple, lightweight but featureful, and easy-to-use e-mail client (mailer, MUA).