I think it is something that would be good to address if we are honest
about how long it will take to get into products. It doesn't need any MUST
or SHOULD. Wording to the effect of "future revisions to CMS SHOULD include
this extension" is appropriate.
I think that your initial draft should also describe an option in
sMIMECapabilities that says "receiver can handle compression types a, b,
and c" so that S/MIME implementations can add this in in an orderly fashion.
--Paul Hoffman, Director
--Internet Mail Consortium