I like how that Mailtrack thing has a “how it works” section on the site and then don’t actually tell you how it works. I’m guessing they embed a breadcrumb in the email which pings back to their servers; if that’s what they’re doing then it’s readily avoidable by the recipient, indeed a lot of modern email clients block this sort of “phone home” behaviour by default now.
There’s no reliable way to know if a regular email has been read (unless you have control of the recipient email server, anyway). There are various ways to tell if an email has been opened but if you get a ‘receipt’ of any form then all you really know for sure is it’s arrived, could be a preview pain or something, you don’t know if it’s actually been read. Conversely, if you don’t get a receipt, it tells you precisely nothing you didn’t already know.
Actually, that’s a lie, there is a reliable method. Contact them via another method (eg, telephone) and ask, “did you get my email?”