Even in the case of Speed Posts or Registered Post or Courier with POD, all it prove is that a closed cover addressed to a particular addressee is delivered. A shrewd receipient can successfully argue in a court of law that the cover did not contain the letter one plead as sent but something else or a blank paper. He will defenitely succeed with such arguements.
How can we prove what we are holding a copy of is what we sent through post and not a blank letter?