Free Gmail tracking with geolocation. With GeoTrack, track if your emails were read, clicked, recipient's geolocation, device used and much more...
Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.
Mailtrack.io - The best email tracking solution I've found. Free
iTrackmail - Want to know if people are reading the emails you send?
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.
Bananatag - Find out what happens to your emails after you press send! Add email tracking, attachment tracking, email scheduling and templates to your daily emails.
Mailbutler - Mailbutler adds lacking functionality to email clients and increases your productivity