Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Outlook4Gmail - Outlook4Gmail provides easy synchronization of your Outlook and Google accounts
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Boomerang for Outlook - Boomerang for Outlook offers email productivity tools to track, schedule and send mails at the right time.
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.
BlueMail - BlueMail is an email management app for Android, iOS, and Windows.