Two additions to Keith's comments, with which I entirely agree...
Some mail systems for PC LANs will gladly accept non-ASCII
characters for user names and mailboxes, e.g. Microsoft Mail for
Macintosh. In non-English-speaking countries such capabilities
will be used.
Yes, but the vast majority of the world can't send them email.
And, to avoid *that* problem, many of the gateways to these systems
support directory-based arrangements that provide an "external" name
(with PrintableString or a subset of it) and map onto an "internal"
name. Model this process a different way and you have the point I was
trying to make yesterday--a username that can be used internally (but
not externally) to access a mailbox whose name is drawn from a more
restrictive domain. That, of course, violates several of Keith's design
principles.
Also note that the requirement Keith calls "Opacity" (and which, as he
points out, is already a firm requirement) prevents most, if not all,
schemes in which either (i) a user types an encoded string for a remote
address which is transformed to, say, 8bit or UTF for the sending-SMTP
to use or (ii) A user types in UTF and the MTA converts it to some
encoded form before opening the connection. You simply cannot assume
that anything used as an introducer-character isn't part of a legitimate
address and, while you might be able to invent quoting conventions for
local use, they would certainly drive users crazy (and violate Keith's
principles d and e).
As part of this, remember that, under normal "one-to-one" mail
circumstances, mailbox names that go into RFC822 address fields are
going off to the MTA to be used as envelope addresses. While we can
fuss with the RFC822 headers quite a lot (although I think we are risky
ground here), the transport address environment is much more constrained
and much harder to change. Long-time participants in this effort will
recall that MIME was shaped in many ways by the assumption that it would
be impossible to round up the MTAs and fix them. If I run a host that
uses extended names, then everyone I talk to has to be upgraded to
understand the conventions, as does everyone who wants to talk to me.
john