Therefore, anyone who sends a private correspondance through a postal system, should have no expectation of privacy. ?
No, you shouldn't. If you do, you're pretty stupid.
The ONLY reason your mail is 'private' in any given postal system is because people don't care about your messages. The cost of looking for something tasty in your message is too high to be worth it.
In an electronic system, with no encryption, the cost is nearly 0, meaning its cost effective to do it to everyone and every bit of data transmitted.
You do not mail 'secrets' around, anyone with a clue knows that. You have your own courier for that if its actually that important.
Most government systems try to maintain a limited level of privacy so that other random citizens don't get easy access to your mail, but its not generally very much effort. In the US its effectively 0 and the only thing slowing people down is the idea that you can get in a lot of trouble for peaking in someone else's mailbox and futzing with their mail. For the majority of the population, stealing their mail is trivial, just open the box.