I don't really see this as a major problem. Just grab a reasonable number of bytes and disconnect. If a user does end up wanting to view the full TIFF, just say "this server is probably stealing your private information and you will also go over your storage quota by loading this image". If the user is OK with that, then download the image again.

99% of the time, the image is just a company logo in someone's email signature and takes essentially zero storage space or bandwidth to retrieve at receive time. For the special cases, just warn the user about the implications of downloading the image, and go do it if they ask.

This is basically the plan. There are some other concerns, notably bandwidth/space and, given it's untrusted input, thinking through all the other ways it could be abused. There will certainly be a byte limit.

Not only would the server need to download the image, it would need to store it in perpetuity because the server would then be responsible for serving them to you (if your client makes any direct request you'll leak when you viewed the email, the trackers don't only track the first request). This is not something that is easy to implement.

A Google security engineer confirmed to me that Google toyed with this idea but ultimately decided not to do it due to various partners not wishing to lose the ability to perform read receipt tracking.

There are no unsolved technical reasons preventing this from being enabled in Gmail, only political reasons.

Huh, I was under the impression that Google started doing this years ago and didn't stop, and even tied it into spam analysis... Did your contact elaborate on the extent of their "toying" and any timelines? I guess I'll have to revisit some of my arguments -- a few years ago I successfully fought against adding a tracking pixel to an email template on the basis of read receipt hits not being valid hits a lot of the time (let alone never getting a hit despite being read due to images being off) due to providers like gmail pre-loading them (sometimes more than once at random times), so we should just use click-through metrics on links when the request comes with an authentication cookie.

Google proxies and caches all external assets in emails through their own servers to hide your IP address, but they only do this on-read instead of on-delivery, which enables read receipt tracking.