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nope, not sure about the gmail proxy servers but you get alerts everytime user opens the mail. speaking from first hand experience.

That happens if email is opened via clients like Thunderbird. For gmail, image will be served via google proxy servers.

What is the cache time for said proxy servers? Does it follow content expiration rules?

In my tests using Gmail website, I see the read receipts right away.

Do you see them the second or third time you open the mail?