Outlook4Gmail - Outlook4Gmail provides easy synchronization of your Outlook and Google accounts
Mailtrack.io - The best email tracking solution I've found. Free
Boomerang for Outlook - Boomerang for Outlook offers email productivity tools to track, schedule and send mails at the right time.
ReadNotify - ReadNotify enables the user to know when email is sent and the reciever reads the email.
BlueMail - BlueMail is an email management app for Android, iOS, and Windows.
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.