An international email address would be portable as far a computers
are concerned; the issue would be for people: how to view it and how
to key it in(as you note). In both cases, that depends more on the
client software than the protocols.
it depends on client software, keyboards, and the ability of the users
to use them. but I suspect that the choice of protocol can change the
degree of dependence on these.
I wouldn't be able to type in an email address in Tamil (though I
could certainly copy and paste it). I would wager that anyone who
cares to have his email used by foreigners would have dual email
addresses, or perhaps even more; e.g. Tamil, Latin, and Chinese.
it's a bit more difficult than that - because email often involves more
than two parties. and it's not as if you can expect those who speak only
(e.g.) Tamil to correspond exclusively with others in that same set.
Keith