Reading this was very surprising for me, because I'd always assumed this kind of thing was impossible.

After going over the comments here, there is apparently a (well known) trick of adding images to email, then tracking hits for that image on your server, thus giving you "email analytics". Apparently this is one of the reasons that most email programs block images by default. In fact, I've long wondered why images are blocked by default, and only now found out.

Always amazes me how many things I have yet to learn!