Mailspring - Mailspring is a mail client for Mac, Windows and Linux.
Followup.cc - FollowUp for email is a lightweight productivity suite that keeps your conversations and tasks from slipping through the cracks. Set perfectly timed reminders that arrive at the top of your inbox, right when you need them.
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.
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.
Boomerang for Outlook - Boomerang for Outlook offers email productivity tools to track, schedule and send mails at the right time.