I agree about the authentication issues.
That said, I have believed for years that senders ought to be
able to assert their opinion of the importance of a message they
are sending, and that receipients should be able filter and
assign actions based on that information, with the understanding
that the filters could apply the recipient's prior opinion of
the priority-settings of the sender.
perhaps, but that strikes me as an entirely different mechanism
(e.g. Importance message header) than one which changes the priority
at which a message is handled by an MTA.
Note that in this scenario, the authorization issue --
"verification of the sender's privilege to set that priority" in
your words --would not apply: senders could make any assertions
they wished, at the risk of undermining their credibility with
the recipient and having their priorities generally downgraded.
right, but absent prior agreement it becomes a guessing game. it's
hard enough to make mail work reliability without asking users to
guess about which priority setting will get their mail through
a particular MTA.
FWIW, I'm not comfortable about the notion of an SMTP extension
that seems to imply utility only for a restricted community
inventing new primary reply codes.
IMHO new primary reply codes should be reserved for extensions that
require explicit support in the client in order to be handled
properly. If an ordinary 500 or 400 will suffice to tell the
client how to handle the message, we don't need to add a new reply code.