What if people want to use their names for logins, and their names happen to
use 8-bit characters?
I think the question really is, "how do we accomodate this?" It can't be,
"how do we prevent this?", as I don't think we can prevent it. Look at the
just-send-8 arguments. We couldn't prevent it, so we invented a way to
accomodate it. That is, ways were invented such that, if you wanted to do
it, there is now a specified way of doing it.
There is a potential mess here, of considerable proportions. There
may also be a way to "accomodate it", but it probably isn't the one that
one might like. Let's keep in mind that the concept of "login [name]"
doesn't need to be bound to "mailbox name": that is just a popular
convention on a lot of systems (although certainly not all). As a
trivial example, we discovered with Multics (and a lot of UNIX systems)
that case-sensitive user names, and even user names that contained
composed (overstruck) characters were locally quite reasonable and much
in demand, but that we needed to contrive to make mailboxes case-
insensitive lest people on remote systems be driven crazy.
If we design systems that allow mailbox names to contain non-ASCII
characters, and require that those mailbox names be used if mail is to
be sent, then the users of those mailboxes will never be able to receive
mail from 7-bit-only SMTP environments, nor from environments that
understand addresses in terms of IA4 or IA5 (e.g., any X.400 system).
Seems to me that would be a pretty big mistake, with most of the problem
borne by the person who chooses to have such a mailbox name. I'd guess
that many of them would rapidly either get over "needing" such a mailbox
name, or would insist on system facilities that permit mailbox aliases
that were plain-ASCII (or IA4 or IA5) and on substitution of such
aliases when mail went off-system.
Getting personal names, or, if desired, login names, into "addresses" is
exactly what the RFC822 "phrase" is about. And that phrase can be
treated as containing extended characters from registered character sets
via RFC1342 [bis]. But it would be really unwise from an
interoperability standpoint to encourage extended characters in mailbox
names. And I think the DNS effectively prohibits them in domain names.