At the moment, MTAs don't look at the
headers much if at all -- just the envelope.
...only use of MARID would be
by an MTA
there are an order of magnitude more MUAs
than MTAs, and MTAs are usually run by people with a greater technical
awareness.
True, but that's got very little to do with anything. If (insert name of
enormous software house here) were to release a service pack, you might see
a huge MUA deployment practically overnight. Equally, when the spamassassin
people make a release, we can see a fairly rapid deployment in delivery
agents, specialised spam filtering tools, appliances and MUA (via existing
tag-based filtering) also.
We can't assume that the only important consumers of this information will
be MTAs.