It's not fool proof but you can do some messing with HTML.
If you include a picture that is fetched from a website when the message is opened in a HTMLified mail reader you can check the web logs to determine which IP read the mail and when. This will also help you work out if the message was forwarded, etc.
Should someone not use an htmlified mail client, or not be connected to the internet when they read the mail, then this will not work.
Personally I have used a single pixel transparent gif in this way to trace people (harassment cases).
Steve