The biggest alternatives I've tried are whole email systems - Polymail and Newton. Newton was bar none the best email handling application AND the best email tracker, however, unfortunately, they went out of business at the end of summer 2018.
Mailtrack is different because it only does one thing, it tracks email. Without Newton (Polymail is also iOS only, and expensive), Mailtrack is really the only decent solution for email tracking without buying a big system like a CRM. I didn't want to pay $100+/month for user seats, I just wanted to track my own emails for some better intel. That's what Mailtrack does, and that's what Mailtrack is priced to do.