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RFC3463, 450 reply codes, and 4.7.1 extended codes.

2005-05-09 06:57:35
It was suggested to me that an SMTP server implementing greylisting
needs to return a descriptive code, and some variant on

450 4.7.1 You've been greylisted, try again later

is the "best fit" from RFC3463, which says:

      X.7.1   Delivery not authorized, message refused

         The sender is not authorized to send to the destination.  This
         can be the result of per-host or per-recipient filtering.  This
         memo does not discuss the merits of any such filtering, but
         provides a mechanism to report such.

Yep.  Stick a 4 on it to tell the other end that it's not authorized *now*,
but might be later, and we're ready to rock.  Unfortunately, there's one
final line to that description:

                                               This is useful only as a
         permanent error.

What's the general consensus here?  Is 4.7.1 in fact useful as a temporary
error in this case, or do we risk the wrath of the RFC wonks if stuff deploys
that uses that?

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