PHPBuilder.com has a new article today that shows how to (almost always) know if someone has read an email you've sent or not via a special sort of return receipt request.
The problem is that depending on the email client software that your recipient is using, he or she will most likely be given the option to suppress that return-receipt from being sent back to you. This renders the return-receipt system almost completely unreliable as a means of detecting when someone has read your email. If you have access to a web server that's running PHP, you can make your own return-receipt system that is (almost) guaranteed to work.
Their method is to embed an image into the email's content. This image is actually a linked image generated by PHP making it able to grab information about the view status of the email it's included in.