EmailTrackerPro - EmailTrackerPro is an email tracking and spam filter solution.
GetNotify - GetNotify is a free email tracking software solution.
Sendia - Send cold email in seconds with Sendia, an outreach tool for Gmail that's free & easy to use!
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.
ifRread - ifRead is a free email tracking service that tracks email you sent.
Mailtrack.io - The best email tracking solution I've found. Free