[JoshTriplett]

Blocking "known" trackers is always going to be a cat-and-mouse game, though. Blocking all 1x1 images and similar would help as well, if you're doing that. But I'd still be concerned about spam that's using remote images for "this email address exists and a human reads it" verification. I'd also be concerned about read-receipt services, especially those that might support self-hosting rather than using a central service that you can easily identify.

[calvinmorrison]

By the time you check if the image is 1x1, you've already downloaded it.

The alternative expensive solution is to download EVERY image a user receives and store it indefinitely. That way the trackers aren't any more useful than "was the email received".

In short, I'd love a better solution to " load all images " vs " load no images " but that's where I ended up talking to our frontend engineer at Fastmail. Obviously he had thought about the problem a lot more then me (I do operations not any frontend) and there's no easy solution.