EmailTrackerPro - EmailTrackerPro is an email tracking and spam filter solution.
Mailtrack.io - The best email tracking solution I've found. Free
ReadNotify - ReadNotify enables the user to know when email is sent and the reciever reads the email.
Sendia - Send cold email in seconds with Sendia, an outreach tool for Gmail that's free & easy to use!
Unlimited Email Tracker - Track opens and clicks, set reminders and delays in Gmail
ifRread - ifRead is a free email tracking service that tracks email you sent.