If by prefetch you mean the server grabs the images ahead of time vs the client, this does not happen, at least on amy major modern platform that I know of. They will cache once a client has opened, but unique URLs per recipient are how they track the open rates.
If by prefetch you mean the server grabs the images ahead of time vs the client, this does not happen, at least on amy major modern platform that I know of. They will cache once a client has opened, but unique URLs per recipient are how they track the open rates.
Apple’s Mail Privacy Protection does this. See https://www.reddit.com/r/privacy/comments/pt9ycv/apples_mail_privacy_protection/ for a post from three years ago talking about it.
I don’t know if any other major providers take this approach but Apple / iCloud is definitely one of them.