Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Shift - Shift is the ultimate email client for managing Mail, Calendar, Drive, Slack, Asana, Jira, Evernote, and all your other favourite apps
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Mia for Gmail - A desktop email client for your Mac, without a browser !
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.
Mailplane - Mailplane is the best way to use Gmail on your Mac. Use an unlimited number of accounts, get new mail notifications, use plug-ins and much much more.