ReadNotify - ReadNotify enables the user to know when email is sent and the reciever reads the email.
Outlook4Gmail - Outlook4Gmail provides easy synchronization of your Outlook and Google accounts
GetNotify - GetNotify is a free email tracking software solution.
Boomerang for Outlook - Boomerang for Outlook offers email productivity tools to track, schedule and send mails at the right time.
EmailTrackerPro - EmailTrackerPro is an email tracking and spam filter solution.
BlueMail - BlueMail is an email management app for Android, iOS, and Windows.