[IanCal]

Images are the clear thing that is typically remote - sometimes to the point of an email that’s entirely just an image, or rather a link to an image.

[mikepurvis]

Doesn't GMail pre-fetch those images and then cache them for the duration of the email being in your inbox, so as to defeat tracker dots, read receipts, etc?

[TheNewsIsHere]

You can, at the very least, register an open by generating unique URIs for any remote content.

I’m not sure how long Google caches it. I know Fastmail is doing the same thing now for any remote content fetched from within their web interface.