ReadNotify - ReadNotify enables the user to know when email is sent and the reciever reads the email.

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.

GetNotify - GetNotify is a free email tracking software solution.

Boomerang for Outlook - Boomerang for Outlook offers email productivity tools to track, schedule and send mails at the right time.

Intelliverse Email Tracker - Intelliverse Email Tracker provides insights to increase the ​productivity of your emails.

Gmelius - Gmelius is the first collaboration platform designed for G Suite and your company's daily tools.