From the mail POV, there are two separate questions:
1. What to do about that header, when passing it as a header to SMTP?
a. encode domain names in IDNA
b. encode local-parts in an encoding that is to be defined, but which
hopefully is very similar, perhaps identical, to IDNA
c. encode human-readable text in RFC 2047
2. What to do about the addr-spec part of it when constructing the
envelope (i.e. RCPT)?
a. encode domain names in IDNA
b. encode local-parts in an encoding that is to be defined, but which
hopefully is very similar, perhaps identical, to IDNA
header and envelope encoding of addresses should be the same.
don't worry about UTF-8 or usenet compatibility anytime soon.
it's far more important to minimize disruption in email service
than it is to use utf-8 on the wire, or to try to keep email and
usenet in sync.
Keith