DidTheyReadIt VS Boomerang for Gmail

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

Yesware - Sell smarter with email tracking and more

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.

Mixmax - At Mixmax, our mission is to change how the world communicates. We’re an email productivity platform that helps sales, customer success, and recruiting teams do their jobs better.

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

FollowUpThen - The simplest way to schedule an email reminder