Hmm. Doesn't this argue that an SMTP extension is the wrong mechanism?
A priority specification can be propogated in a header field end to end
even if intermediate MTAs don't support it. If you use an extension for
this all the intermediaries have to support it.
yeah, it's a tradeoff. priority is definitely an MTA function, and for
the past few years we've been fairly strict about requiring that MTA
protocol be transmitted in SMTP. also, if a message is resent then
it will keep the same headers but should not necessarily keep the same
The counter is that in order for this to be recipient-specific it has to
be an SMTP extension.
and that's another point.
the other point that comes to mind is that if a UA doesn't submit
a message via SMTP (or SUBMISSION) then it doesn't have a good way to
set delivery priority unless it's via a message header.
and if the purpose of priority is to make better use of limited resources
then you want to be able to set it on a per-message basis, not have it
derived from the sender or recipient address and MTA-local rules.
Additionally, given that MTAs try to avoid splitting messages, it isn't at all
clear how to handle the case of two recipients with different priorities that
are travelling together. At a minimum some guidelines need to be given for
I'm not sure how this is very different than an MTA that tries to relay
a message to two recipients at the same server (or at different servers),
and RCPT gets a 200 reply for one recipient and a 400 reply for another.
The message doesn't get "split" but one recipient in the envelope
is marked as delivered and another one is marked "try again later".
And I don't think there's any requirement that messages with lower priority
be given worse treatment than a message with high priority. So an MTA
that treated every recipient as if it had the priority of the highest-priority
recipient in that envelope would still be conforming (if somewhat pessimal).
Certainly if I could relay the message to other lower priority recipients
in the same transaction as a higher priority recipient, I'd do that.