Mailbutler - Mailbutler adds lacking functionality to email clients and increases your productivity
Mailtrack.io - The best email tracking solution I've found. Free
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Unlimited Email Tracker - Track opens and clicks, set reminders and delays in Gmail
myMail - myMail is the smart email app for managing all your accounts on iOS and Android.
MailTracker - MailTracker is a mobile email tracking solution.