That is an interesting idea.
I haven't tried it but maybe use an onload event in the email that runs some javascript the sends data via AJAX back to the database. Of course many email clients may block the javascript but you can try.
Either that or you can try a feedback link in the email that goes to a web page with a quick 3 or 4 questions.
I know I have gotten emails that have received conformations that pops up for me. I would guess there is a way to use that feature in PHP generated emails. But I think the confirmation would go to your email so you would have to use another tool to intercept it and store it in the database.