Hi etron,
you are right this can be done with the header. I checked with my Email-program and it includes the line
Disposition-Notification-To: "Myself" <me@myself.com>
that asks the reader to send a notification when he opens it. This is the only way to get an information that he opens it. If he does not want to let you know, you will not get an information.
When you receive the notification, you can find out to what email it corresponds with a message id, that you should set in the header when you send it. This can be something like
X-MyMailID: 12345
To read the received notifications you can use the fsockopen-command for your pop-account. A documentation for that can be fount in some faqs.
Good luck
Grisu