Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Geary - Geary is an email application built for GNOME 3.
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.
Nextcloud Mail - 💌 Mail app for Nextcloud
Gammadyne Clyton - Clyton is an email client with advanced security and spam filtering features.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!