There is a hack to do this, the infamous email bug. Warning: I've never used this, and I'm not recommending it. It's used by many spammers to check whether some email addresses are active or not, I'm not sure whether it's still being used: if your receipient allows html in their email, you can embed a bug in the email (basically an <img/> tag to point to a logging script on your http server), whenever the user opens an email, your logging script will get a hit.

In reply to Re: request a return receipt by johnnywang
in thread request a return receipt by bobdole

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