Mailtrack.io - The best email tracking solution I've found. Free

EmailAnalytics - EmailAnalytics is an email analytics and statistics solution for Gmail​ and GSuite.

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

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

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.

CouponFollow Cently - Coupons at Checkout is a browser plugin for Chrome, Firefox, Internet Explorer, and Safari that...