> Ned Freed <ned.freed@xxxxxxxxxxx> wrote: > > > > Wrong group of codes. Those are status for mail systems to return, not > > > > the routing layer. > > > > > The point of null MX records it to explicitly say the address is invalid, > > > so an address status would seem to make sense. > > > > No, the point is to say that a host is invalid. > If you can't use them when the DNS says a mail domain is invalid then I > don't understand if it ever makes sense to use 5.1.2 or 5.1.8. You don't have a way of declaring a domain as invalid at the MTA level? The Oracle MTA has at least five different ways that can be configured without any recourse to a directory server. And we absolutely do return 5.2.1 in such cases. And ask yourself this: Given that the null MX mechanism wasn't part of the picture when these codes were developed, and given null MX is the only (AFAIK) DNS-based mechanism with these semantics, why was the code even defined? Also remember that the attitude in those days (wrongly, IMO) to be parsimonous with code allocation. Absent a DNS-based mechanism, the best that could be done to address this problem was either to MX such hosts to an MTA which was configured to reject them, or run a stub SMTP server on the host itself that returns a 521 error as the banner a la RFC 1846. In either of those cases a 5.1.2 code would be appropriate. Ned