Funny enough, this is the basis for one of David Copperfield's magic tricks. He sends the audience an email before the show starts, implores them to not open it (and I'm sure if they open it, it'll be blank or whatever). Then during the show, the audience comes up with a word that Copperfield could not have know. At the end of the show, he asks you to check that email that was sent long before the word was picked. Of course the word is "magically" there. In reality, the email was sent ahead of time with a linked image. The image contents at that URL are hot-swapped during the show to include that word... "magically".

Which is why email should not have linked images.

It's too bad that mail clients encourage people to write HTML email by default. The default should be plain text, or perhaps better yet, text/markdown but without links as image sources.

Which is one of the several reasons why I don't allow my mailreader to resolve any external links.

Lots of mail hosters, like gmail, have been proxying and caching image requests for a while now.