BIND vs. sendmail problem

[Michael Kohne]

I'm trying to debug a BIND vs. Sendmail problem for the office. If it were
just inter-office e-mail, I'd probably not bother to push these folks into
fixing their stuff, but as it seems to affect our customers as well, I feel
I'm forced to act. 

The problem manifests as a nameserver timeout to sendmail. 

<xxxxx at xxxxx.xxx>... Deferred: Name server: xxxxx.xxx.: host name lookup
failure

Things I've checked and had them fix:
Their NS records were previously pretty screwed up. Some of the servers
didn't answer DNS, some didn't claim authority. This is all fixed now, and
I just checked - it all matches the NIC registry as well.

Now, the only odd thing that I can see that is odd about their setup at the
moment is their MX record. 

Results of some tests:
nslookup xxxxx.xxx 	works and gives the correct address

nslookup -type=mx xxxxx.xxx   fails with '*** localhost can't find
nanoose.com: Non-existent host/domain'

dig @<their nameserver> xxxxx.xxx any
<abridged-cut out NS and SOA>
xxxxx.xxx.            1D IN MX        5 xxxxx.xxx.
xxxxx.xxx.            1D IN A         nnn.nnn.nnn.nnn

dig xxxxx.xxx any
<only contains A and NS records>



The thing I think is odd is that the MX record just points back to the same
name. Now I know that MX records aren't supposed to point to CNAMES, but
what about this situation? I'm thinking that it's illegal and screwing
things up royally. A quick dig of several other domains (including other
offices of the company, and some popular sites like yahoo) indicates that
no one else seems to do this. 

Am I correct? Is this MX record illegal, at least as far as sendmail is
concerned? If so, why? Is sendmail doing a query that will return both
types of records, and therefore getting the MX again? Is the DNS server on
the other end actually screwing up because of this? In addition to an
answer, I'd love to know the reason for the failure, so that I can more
easily convince the other end to fix things.

Secondly, is there a list of 'MX' rules that will tell me what's legal and
what's not?

Any help that anyone could give would be greatly appreciated.



As a side note: I do not believe that the problem is sendmail on my end,
only because I don't have trouble sending mail to anywhere else, and
because some of our customers also have trouble sending mail to those
folks. If there is some way I could have misconfigured sendmail, I'd love
to hear about that too.

Thanks!

Michael Kohne
mhkohne at discordia.org
"Evolution is God's version of Domino Rally"