We used to (and I believe still) have Read Receipts for this.

Yes! +1 And it's possible to disable Read Receipts and a lot of people do. A lot of people mangle beacon pixels too.

read receipts just mean you opened the message, not that you actually read it. It's a terrible misnomer.

Oh yeah, the thing I always turn off or filter out because the last thing I need is another way spies and hackers and ad men can track every waking moment of my life.

Read Receipts are invasive, because they signal when I read it - which might well not be when I want to reply. I could read an email today and then decide I'll reply tomorrow or next Wednesday, because reasons; but the sender will see the read receipt and just assume I hate their guts for not replying immediately to the Most Important Person In The World (i.e. them).

So I turn them off as a matter of routine on all my accounts.

That was another groupware feature that was great for small orgs and terrible for the general net.