Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
The Bat - The Bat! is a secure desktop email client for Windows, designed to protect your correspondence against third-parties and to save your time.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Mu4e - Starting with version 0.9.8, mu provides an emacs-based e-mail client which uses mu as its back-end: mu4e.
Nextcloud Mail - 💌 Mail app for Nextcloud
Discourse - Discourse is an open source discussion platform built for the next decade of the Internet.