I remember when the email
network was a heterogeneous network consisting of UUCP, BITNET, DECnet,
SMTP, X.400, and a few other things thrown in. It "worked", sort of,
but we had all kinds of problems with the translations at the boundaries,
with addresses from one network leaking past the gateways into another
network, with addresses being "translated" in such a way that they
were no longer usable in the destination network.
There was even an analogy to NAT's "addresses embedded in the application
data stream" problem: if you had an address in your .signature, the gateway
couldn't translate it, so the person receiving your message saw an address
they couldn't use.
at least in those days, gateway proponents didn't insist that people
shouldn't include email addresses in the bodies of their messages.
Keith