Second, you're failing to consider that there's essentially no incentive
for a significant percentage of the world's mail users or MTA operators
to upgrade. An approach that relies on MUA upgrades to provide the new
functionality lets those who benefit from the new functionality - those
who have an incentive - do the upgrade and get immediate benefit. This
gets it deployed more quickly for those users who need it.
I think this is the key issue.
People in the UK, USA (and a large percentage of Europe) won't want to
fiddle with things that ain't broke just so that some people can send them
emails that they can't understand... This means that the majority of
Internet users will be running mail systems which people using IDN
wouldn't be able to contact in the near/mid future.
actually, they'll be able to contact each other. it's just that the IDNs
will be displayed in encoded form on the systems that haven't been upgraded,
and the user with the non-upgraded MUA will have to type in the ugly encoded
form of the address (or copy-and-paste that address into his address book).
(as a practical matter, folks who often correspond with speakers of
other languages will probably want to have an easy-to-type email
address - e.g. one consisting of a short sequence of ASCII-only characters.)
All non-latin email addresses/domains would either have to have a latin
equivalent, or would just have a scope of the country of origin. If all
addresses have to have a latin equivalent, then, simply, why not use that
for all the 'important' stuff, and just use the local script for display
purposes.
if you substitute "ascii equivalent" for "latin equivalent", that's
essentially what IDNA does.
Frankly, I don't think the demand for non-ascii addresses is from folks
who want to use such addresses to communicate with speakers of other
languages - I think it's from people who want to use them when
communicating with speakers of their own language.
If you want Chinese (for instance) emails just to have a scope of
China, then why not have a new set of protocols ('UDNS', 'USMTP' etc) which
handle Unicode addressing, with gateways run by ISPs in those countries to
convert 'USMTP' to SMTP to go to the rest of the world.
arrgh! sure, let's build an entire mail system from scratch and run two
incompatible mail systems just so that half of the users can have one
additional feature. makes *lots* of sense...
Keith