well if you just wanted to know if someone read or opened your email, there are services like didtheyreadit.com and toutapp.com and that will track this info for you, sort of like a receipt. If you're looking for a quick hack(limited results), you would embed an image in the email and track how many times it is accessed. And most email marketing software track this information for you. That solves the problem of "Is there anyone there", but not the getting the person to reply problem.