Currently Being ModeratedMay 31, 2012 12:50 PM (in response to gumsie)
I'll answer your question, not because I think this is something you should do, but so that others will know how it's done and can guard against it. If you want to know whether the recipient has read your message, you should ask him for an acknowledgement, instead of trying to force him to acknowledge whether he wants to or not.
An HTML message can contain a link to an image, which might be an invisible 1-pixel "web bug," hosted on a web server controlled by the sender. The image is unique -- only linked to that message. When the recipient opens the message, his mail or web client requests the image, and the web server logs the request. The sender reads the log and knows when the message was opened, as well as the recipient's IP address and maybe other information.
That's one reason why mail should not be read in a web browser, and HTML images should not be loaded automatically in a mail client.