as SirDice noted, if you is able to send HTML to the target, you can just generate unique objects (e.g. .GIF) that is referenced by the e-mail body. When the user open the e-mail, a reference is made to the .GIF (one for each e-mail) and you know that user had opened the letter.
However, it is not 100% perfect. There is tools to block this kind of trick, considered by some guys an "spyware". Or if the target downloads the e-mails, disconnect from internet an open the e-mail, no notification will be sent back.
You can be 100% sure that the guy really read the e-mail BUT
you cannot be 100% sure that the guy didnt read the e-mail.