They already handle MIME.
Some do, some don't. ...
This issue of backwards compatibility has the character of
dogma. John's right, of course, we need experiments.
Actually, the question of whether MIME is supported seems to me
to be irrelevant.
Any MIME-based scheme is going to have visible impact on the
content. Certainly using s/mime is going to have directly
visible impact on recipients and forwarders.
That's why the requirement is best stated in terms of impact
rather than debating how many mime muas fit on the head of a pin.
A sub-observation is that MTAs are going to have to make
changes to support MASS regardless. Adding MIME to that
however mass is not specific to mtas.
and the impact on non-adopting mtas and muas is the concern, not
the effort need to adopt.
dcrocker a t ...