I mean, considering all of the problems that SMTP has -- sender forgeries,
relay forgeries, lack of end-to-end integrity, and of course a lack of
support for i18n email addresses -- it seems likely to be a candidate for
replacement for other reasons anyway. That doesn't mean that the IDN tail
should wag SMTP, but if SMTP cannot be upgraded, well, add that to the
list of other things that SMTP doesn't do very well. In this regard, the
IDN issue is just one of many that need to be considered (i18n mailbox
names being another).
if you can somehow solve the PKI problem, you can make SMTP immune to
forgeries and have it provide end-to-end integrity. so far, nobody
has managed to do this.
if you can somehow figure out a way for anybody to type in a mailbox
in any language on any keyboard, you can solve the i18n mailbox problem.
until then, there's very marginal value in replacing SMTP. even then,
it would probably be easier to upgrade SMTP than to replace it.
Keith