Ned Freed <ned(_dot_)freed(_at_)mrochek(_dot_)com> wrote:
What's wrong with 5.1.2 and 5.1.8 ?
X.1.2 Bad destination system address
The destination system specified in the address does not exist
or is incapable of accepting mail. For Internet mail names,
this means the address portion to the right of the "@" is
invalid for mail. This code is only useful for permanent
failures.
X.1.8 Bad sender's system address
The sender's system specified in the address does not exist or
is incapable of accepting return mail. For domain names, this
means the address portion to the right of the "@" is invalid
for mail.
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.
X.1.XXX Addressing Status
The address status reports on the originator or destination
address. It may include address syntax or validity. These
errors can generally be corrected by the sender and retried.
An invalid address isn't a problem with the DNS itself.
Neither is a NXDOMAIN error, and that's in the 4.z group.
X.4.XXX Network and Routing Status
The networking or routing codes report status about the
delivery system itself. These system components include any
necessary infrastructure such as directory and routing
services. Network issues are assumed to be under the control
of the destination or intermediate system administrator.
Looking through the X.4.X codes, I can't tell whet is the distinction
between 5.1.2 and 5.4.4. The text for X.4.4 has a weird description of a
nodata / nxdomain response, which is exactly the same as "does not exist"
under X.1.2.
Again, it's a question of what component is involved. 4.x codes are
for directory services external to the mail system, 1.x is for the mail
system itself.
Which admittedly leaves LDAP in a bit of an odd position. Oh well.
A DNS lookup returning only an SOA (Start of Administration)
record for a domain name is one example of the unable to route
error.
There's evidently a lot of overlap and lack of precision in how these
codes are defined and used so I don't think it matters in practice what
this draft picks.
On the contrary, it matters because the DNS changes constantly, and
the code may be all you have telling you what the condition was that
caused the bounce.
Ned