[have_faith]

>gmail let's this behaviour

I'm assuming the server is just responding with a different image depending on a query param embedded in the image url? (an old technique), what should google do? any remote image url could respond with a new image in an old email it's just rare that it happens.

[reader_1000]

It used to prefetch external images [1]. Another option would be asking whether to download external images. I think one can enable this in settings, default is always display external images.

[1] https://arstechnica.com/information-technology/2013/12/gmail... [2] https://news.ycombinator.com/item?id=6896378

[have_faith]

Yeah I always have all images disabled by default and turn them on on a per email basis if it's absolutely necessary. 90% of emails don't need them or just contain tracking pixels.