Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Geary - Geary is an email application built for GNOME 3.
Mu4e - Starting with version 0.9.8, mu provides an emacs-based e-mail client which uses mu as its back-end: mu4e.
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.