GMail does. It bounces the images via a server to strip out IP info but it opens them.
AFAIK all webmail providers do too, otherwise there wouldn't be so much of a market regarding email marketing with open rates and remote updating content (eg https://movableink.com/gallery/autotrader)
I'm honestly confused that Gmail would proxy their images to anonymize them, but wouldn't just make that proxy a caching proxy, such that a given image URL would only ever get one retrieval from Gmail's servers. It'd be a huge win for them bandwidth-wise.
> I'm honestly confused that Gmail would proxy their images to anonymize them, but wouldn't just make that proxy a caching proxy,
They do that, but images in mail have unique URIs per recipient to enable this sort of tracking. Before Google enabled caching, it was possible for sender to figure out the exact number of times an email was read, now you can only know the first time it is read.
AFAIK all webmail providers do too, otherwise there wouldn't be so much of a market regarding email marketing with open rates and remote updating content (eg https://movableink.com/gallery/autotrader)