I'm not convinced that this would be as big a problem as you're suggesting. If somebody is sending thousands of emails with html containing embedded images, the

I'm not convinced that this would be as big a problem as you're suggesting. If somebody is sending thousands of emails with html containing embedded images, the receiving system of those emails only needs to download each of those images once, and cache them. Unless they're using different URLs for each image for tracking purposes. Which would become pointless if the images were downloaded immediately. They could probably get away with applying size limits to images too, and simply have placeholder/broken images if the images are unreasonably large. It would even work for people using normal IMAP clients if they attached the images to the email directly and switched out the href in the img tags to point at the attachment instead. This whole thing could be a user setting which can be toggled. Defaulting to the most privacy friendly option.