I think the problem here is the characterization of this proposal
as an end-to-end QoS-assurance mechanism rather than just a way
that trusted and authenticated parties can request priority
handling of important messages through those parts of the mail
system that trust their credentials and are willing to do that.
If we're trying to do the end-to-end QoS, I have huge doubts about
our being able to make that happen. We'd not only have to
rigidly define the various levels of service, we'd also have to
solve the general cross-realm trust problem. (And then we'd have
to get people to deploy something that they probably couldn't
understand how to use.)
OTOH, If we're just trying to design a standard interface to a
general-purpose mechanism for requesting priority handling
among mutually consenting parties, that's much easier, and IMHO
quite doable.
Keith