The extension produces a potentially useful optimization, but there
seems to be a lot less interest in doing that work than there is for
doing the BCP. It's not clear this is a major pain point that's worth
the significant effort of producing a standards track extension and then
expecting industry to begin adopting it.
The whole point of greylisting is to leverage something that
legitimate MTAs do anyway. If we're going to add an extenstion to
tell MTAs how to look legit, I'd rather add "sign all your mail with
DKIM" than "try in five minutes."
It would probably make sense in a greylisting BCP to document the
retry times that a greylist is likely to see (occasionally a few
seconds, more often a few minutes to an hour, the old sendmail
interval.) That doesn't change any specs.
R's,
John