You could probably detect the message being opened if you wanted to. I think I have read somewhere about RFID to see if someone has opened a marketing mail, and

You could probably detect the message being opened if you wanted to. I think I have read somewhere about RFID to see if someone has opened a marketing mail, and I have also read about using eggs to seal the message to make it harder to open without destroying it entirely, as well as reusing the envelopes from junk mail to send your own messages. There is other stuff too possible such as trap messages I suppose. Also, the message may be hand written and is not necessarily typed, so hand written message would probably make OCR more difficult I should think.