There is another way, send the email in html format and have a 1x1 pixel transparent image that is stored on a web-server that you are in control of. When the email is opened, the email client will attempt to download the image (depending on the mail client configuration, but many people don't lock down their mail applications from preventing this trick from working). Make sure the image has a unique name and don't allow anyone else to know the name of it, so that way you're sure who opened it. You'll even get the guy's IP address
Dave.