I have often thought about this idea (but never got around to doing it!) The problem: how do you know if an email you’ve sent to someone has been read (and if so, when)? WSJ describes the solution:
Enter MessageTag, from New Zealand software-development company eCOSM Ltd. Install the MSGTAG software and, in most cases, it will automatically reconfigure your e-mail software to add a glob of code to the bottom of any e-mail you send (to those of you in the know, it’s an HTML image reference) which assigns the e-mail a unique ID number. When the recipient opens their e-mail, the glob of code sends a message back to the MSGTAG server, or computer. That computer makes a note of the ID, and the time the message was received. It then matches the ID with the MSGTAG user, and the matching e-mail, and notifies the user the e-mail has been opened, and when.