Some brainstorming: - Could be used for entrapment, that is to say, could send over potentially criminal information and argue that the user did in fact "recei

Some brainstorming: - Could be used for entrapment, that is to say, could send over potentially criminal information and argue that the user did in fact "receive" it since it was read. - If you could inject your own tracker into someone else's email, you might also gleam when and potentially how they read that email. - One funny thing that came to mind. In Australian contract law (likely similar to the UK from which the laws were adopted), if you email someone an your acceptance to a offer, that acceptance is deemed as received when the email is read (the person becomes aware of the acceptance). Where as with regular postal mail, the offer is accepted when posted (the "postal rule", a nice headache). If you had a tracking code, it would be harder to to argue you weren't aware of the acceptance by email. That is unless your email client pre-fetches images before the email itself is opened...