These include the number of times a recipient opened a certain mail, the gap between each of those openings, reply timing, etc.
MailTag
MailTag is one of the few open tracking tools that also gives you a high-level look at your overall open rates, link-click rates, and more.
Banana Tag
This email tracker software makes it easy for you to track your emails individually. With plugins for Gmail and Outlook, it makes it a breeze to track your open rate and engagement.
Snovio
This free email tracker software enables you to switch between different Gmail accounts. You can also switch it off whenever you want in the Google Chrome extension.
Autoklose
Autoklose is one of the email tracking tools that can change the way you track your KPI opens, rates, clicks, and more.