I was using Mailtrack for 2 years. It has some limitations for the free version. For Example, receivers would notify that this email is tracking. Also, it has limitations for the number of tracked emails and it doesn’t show you which of your recipients opened the mail.
Moreover, I had to use another extension to schedule and notify unread email. I chose Boomerang for Gmail. And it had limitations like Mailtrack.
Recently, I found Unlimited Email Tracker. It’s a Mail Tracker and Also a Mail Scheduler. Gives you notifications for unread emails. And, receivers won’t be aware of it! Its Totally free and doesn’t have any limitation.
If you are a Gmail user, try it!