4 ms·

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

• pixl97 6y ago
  What is the cache time for said proxy servers? Does it follow content expiration rules?
• sroussey 6y ago
  In my tests using Gmail website, I see the read receipts right away.
  • bigiain 6y ago
    Do you see them the second or third time you open the mail?