Having thought about this a bit, I can't see any problem with a BCP or
even with the server saying 'try again in 5 minutes' (or whatever)
Yes, you may say that this will help spammers - well, if spammers are
able/willing to 'try again in 5 minutes', they will already be doing so,
thus getting around greylisting. If they aren't (for whatever reason),
then they'll probably also ignore the 'try again in 5 minutes' response.
They obviously know about greylisting, so apparently many have decided
not to bother with it for some reason. Why would telling them the retry
time make them change their minds about this?
However, the 'try again in 5 minutes' information will help legitimate
senders/receivers who want to take advantage of the feature, so it has a
definite up-side, and a dubious down-side. As previously mentioned, the
'try again in 5 minutes' response doesn't need to be limited to
grey-listing but could be used for load management as well.
It wouldn't even need to be much, just respond
421 4.2.x [retry:300s] connection refused for now. Try again later
it wouldn't interfere with MTAs which don't understand it, and would
give a hint to those which do.