Need for ESMPT extension for IMAA?

In John's proposal for a very different idea of how to do IMAA, he said:

At 8:00 PM -0500 2/14/03, John C Klensin wrote:
>	Loose end a.3: Because of the opacity requirement, I
>	believe that even IMAA would ultimately require an ESMTP
>	extension and negotiation to work properly.  So, in some
>	ways this proposal and IMAA are compatible and
>	complementary if an IMAA ACE string is used rather than
>	UTF-8.   I think that would be overkill unless we really
>	like that hammer.

Could you elucidate? I cannot see how IMAA touches standard SMTP at 
all if we agree (as I'm pretty sure we do) that the local part is 
opaque.

--Paul Hoffman, Director
--Internet Mail Consortium