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.
Postbox - The Power Email App for busy professionals, like you.
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.
Claws Mail - Claws Mail is an email client (and news reader), based on GTK+, featuring Quick response