We can't assume that the only important consumers of this information will
be MTAs.
Agreed, not hardly. I think that enforcement of policy will end up
being in the MTA realm (because changes in policy there affect entire
user-bases, not just single mailboxes, and it's one place where
bandwidth usage really hurts). Joe-jobs will be reduced.
I think that once a standard (or even a few de-facto standards) appears
and is fairly widespread, we'll see MUAs grow interfaces for showing the
information. Phishing will get harder. A "trust" column might appear in
a MUA interface. Web links in suspect emails could be sanitized by
default.
Ari