Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Spark Mail - Spark helps you take your inbox under control. Instantly see what’s important and quickly clean up the rest. Spark for Teams allows you to create, discuss, and share email with your colleagues
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Newton Mail - Newton is a fully featured mail client available for Android, iOS, macOS and Windows.
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.
Aqua Mail - Android Mail Client.