In the UK, as I said. I would think that other countries may have
a similar system. Note that this is a natural example of NAT,
in which the post office is doing the address translation to a local
address that only that post office knows, but which is globally
reachable through that post office. And the post office does so
without changing the global addresses or the local addresses.
I think the example you give is more like ARP or VLAN than NAT.
If the postal service were NATted, you'd send your mail to the post
office, the mail clerk would decide that you really intended it
to go somewhere else, and would erase your original destination
and return addresses and fill them in with something different.
Any address that you actually put in the text of the message would
be useless to the recipient. Similarly, business cards, telephone
directories, or any other means used to look up addresses outside
of the postal service's control, would be useless. Each post office
would need to have its own telephone directory for every telephone
with which that you might want to call, so that you could look up
a telephone number using your local post office's spelling of the
address. If you moved from one place to another, such that you were
now using a different post office than before, you wouldn't be able
to continue using snail-mail to correspond with anyone with
whom you'd previously been corresponding, because you would no
longer have a usable address for that person.
Keith