--On Friday, August 15, 2008 6:07 PM -0700
ned+ietf-smtp(_at_)mrochek(_dot_)com wrote:
Unless it did so as one of the "shutting down service" or
equivalent messages. I.e., 450 might make sense, other 4yz or
5yz codes would be very unusual.
452 insufficient reousrce sorts of codes are also a
possbibility here.
But I would stand by "unusual" in the sense that an insufficient
resource status ought normally to be detectable, and probably
should be reported, earlier. I did not mean to suggest it
couldn't happen (in particular, race conditions are clearly
possible) or that the standard should prohibit those other codes.
Of course, if there were no
valid recipients, even "command out of sequence" (or
equivalent) would be plausible.
Other permanent failures are also possible, e.g., various
things along
the lines of "Something you have said caused me to hate you
and I'm rejecting this tranaction in toto".
Which, again, would "usually" be delivered after MAIL or some
RCPT command ("if you are trying to send to _her_, I hate you
and...").
The unusual and edge cases are why there is so much flexible
language in 2821/ 2821bis and I would resist removing that
language --or inserting new rules to badly constrain the cases
that require the flexibility -- as, or more, strongly than you
would.
john