Boomerang for Outlook - Boomerang for Outlook offers email productivity tools to track, schedule and send mails at the right time.
GetNotify - GetNotify is a free email tracking software solution.
Outlook4Gmail - Outlook4Gmail provides easy synchronization of your Outlook and Google accounts
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.
Covideo - Covideo is a Video Email Platform for Businesses that enables companies to communicate quickly and effectively with their customers.
Mailtrack.io - The best email tracking solution I've found. Free