Ned Freed <ned(_dot_)freed(_at_)mrochek(_dot_)com> 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