Occasionally, the Other Crocker ventures into email space and he chose to be
more thoughtful about a brief thread that was just on the IETF listthat noted a
message delay due to being held for moderation.
Currently, nothing in the message (or smtp responses, of course) signals this
reason for delay. (For those who haven't seen the thread, the person initiating
the the thread thought the delay was due to DKIM processing...
It would be quite simple to add a Received header field, when a message is
placed into a moderation queue, noting that fact.
Given the history of trying to get mailing list software to support common
mechanisms (standards?) this might be challenging to pursue, but I thought it
worth raising to the group.