The Post Office can’t tell you if the recipient opened your letter—just that they received it. (And I don’t think we would want the Post Office to know which letters you read and which you junk, either!)
So your old process of checking the email server logs seems more analogous to Post Office tracking to me. Would it be possible to automate that?
In that analogy, it would be more like you can only know that your local post office delivered it to their local post office... you have no idea if the carrier actually delivered it to the mailbox, or if it is just sitting in their sorting room still.
The app does tell the user their document was successfully delivered or not as well. It also counts the number of times an invoice has been opened. I could also track the date and time of that but I don't.
So your old process of checking the email server logs seems more analogous to Post Office tracking to me. Would it be possible to automate that?