I said this to Jeff in person at IETF but here it is for the list:
It's my understanding that this was based on a collection of various SMTP text
replies being returned by various ISPs when objectionable content is presented
for delivery. Some similarities were noted, and then some consolidation was
attempted and aligned using the proposed x.8.y codes. There is to date no
actual implementation of this, and no information about how agents that observe
these codes during SMTP transactions (i.e. senders) would respond to them.
That is to say: There's no implementation at either end yet.
I think that's fine for Informational or Experimental, but this draft is
seeking Proposed Standard status. I'd like to see the results of some
implementation experiments at least.