How do you send a message such that the sender gets a return receipt when the recipient opens the email?
John Zukowski (Thankfully) This isn't part of the SMTP mail protocol. It also is not possible given the way e.g., security and mail relaying work. I.e., intermediate relays have no clue whether or not a particular mail address is real or not or just another forwarding alias or a mailing list or... And, any decent mail server won't tell you if the email address is real or not since that's a security hole. In terms of robustness, this return-receipt concept is silly, check out: ftp://koobera.math.uic.edu/www/proto/nrudt.txt for a more thorough explanation.
However, RFC 1891 does define a way to find out if the message was successfully delivered. See How do I receive an acknowledgement that a user has received my message? for information on its usage.