With the ever-increasing use of HTML email, senders can embed images into their emails. These can then be loaded from a remote web server or attached directly to email. When images are remote, they can be tagged by the sender to know who, exactly, opened the message and when. Giving them, in effect, a return-receipt capability that can't be disabled by the recipient. Here's a safe demonstration of this concept. Luckily, I loathe HTML email so I have it disabled anyway.