Or I guess the mail server could just always precache all images at the time of receipt. Then nothing new happens when the mail is read. I think somebody here is describing GMail's approach that way.
Then email servers would have to download billions of images of which 99% the corresponding email won't even be opened
gmail's cache hides a users location but you can still tell when they've opened an email, and you can still get a ton of information if they click a tracked link within the email as well.