It would be nice if the server could say "give me a minute to get my stuff together and come back", have the client retry in 60 seconds and get the mail accepted for delivery - with a delay of a minute or so, rather than a delay of 30 minutes. The server gets to shed load, the client gets to dump a message out of it's queue and the human correspondents see fast, in-order mail delivery rather than slow, out-of-order delivery.
That's the end-user visible part of the thought behind https://datatracker.ietf.org/doc/draft-atkins-smtp-traffic-control/ anyway.