From time to time we have the problem that a mail posted to a list triggers an autoresponse (vacation, bounce, tdma) from an unexpected source. Sometimes it isn't possible to identify the subscriber who is causing this. One example for this was "petsupermarket".

So i wrote a small tool, which takes Mailadresses as input and resolves the domains until it reaches IP-level, so it is possible to identify "related" addresses to such an incident.

I now ran this tool for all 38656 domainparts mailadresses that are currently subscribed to some list at our listservers:

Unresolveable Domains : 443
Grounded Domains : 3
Domains with A : 1058
Domains with CNAME, A : 59
Domains with CNAME, CNAME, A : 3
Domains with MX, A : 36202
Domains with MX, CNAME, A : 1199
Domains with MX, CNAME, CNAME, A : 22
Domains with MX, CNAME, CNAME, CNAME, A : 5
Domains with MX, PTR : 36
Domains with unresolvable Hosts : 361

(You may have noticed that those lines doesn't sum up to 38656. It is possible to have more than one MX-Host, and it is also possible to have more than one IP in an A-Record. If one of those combinations falls into another category it is counted twice)

so we have 36528 domains that are completely configured correctly according to RfC 2821 with a MX-Record pointing to an A-Record, or without a MX-Record and an A-Record. These are 94.5% of all domains.

but we have 443 domains in our list, that didn't resolve at the moment i ran my script, those have to be investigated.

We also have 3 domains that are configured with 'IN MX 0 .' as decribed in this expired Draft, those also have to be investigated, and thrown out.

We have 1288 domains, which use CNAMEs (or CNAMEs pointing to CNAMEs) in their MX or directly on their domainname. RFC1034 says:

Domain names in RRs which point at another name should always point at the primary name and not the alias. This avoids extra indirections in accessing information.

then there are 36 domains which point their MX directly to an IP-Number. RFC 1035 says:

3.3.9. MX RDATA format

+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| PREFERENCE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ EXCHANGE /
/ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

where:

PREFERENCE A 16 bit integer which specifies the preference given to
this RR among others at the same owner. Lower values
are preferred.

EXCHANGE A <domain-name> which specifies a host willing to act as
a mail exchange for the owner name.

MX records cause type A additional section processing for the host
specified by EXCHANGE.

So according to that an MX has to point explicitely to a Full Qualified Domain Name, and it has to be an A-Record it points to.

However: Most MTAs (including ours) these days forgive this, and figure out the right thing.

At last: we have 361 Domains which MX and/or CNAME-Records point to Hostnames that are currently unresolvable. A quick check shows that often the problems only appear on one MX-Record, while another is correct, so the service is functional.

So maybe now it is a good idea and check your own DNS-Setup and the Mail-related Data.