Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Geary - Geary is an email application built for GNOME 3.
Nextcloud Mail - 💌 Mail app for Nextcloud
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.
Mutt - Mutt is a small but very powerful text-based mail client for Unix operating systems.
Gammadyne Clyton - Clyton is an email client with advanced security and spam filtering features.