The easiest way that I know of is to send the email as HTML and include a line to open a small image on your own webserver. I suppose you could also just add their email to the end of that url as a get variable. Then just parse your logs for requests for that particular image, and there you go... You know that it was opened, by whom, and at what time.
Ofcourse all that is assuming the reciepient is using an HTML capable mail client and that they haven't blocked their client (as I have) from accessing anything though http.
