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