Insight on MX-Entries

Recently, I gained some insight on the intended nature and its implications of the interaction of DNS with mail services.

It seamed that rather suddenly, my mail server was no longer able to receive mails from certain mail service providers like web.de. Those sending mail servers were only little verbose about what the source of the problem was:

domain has no mail exchangers

That was rather strange to me as I had some good experiences so far regarding the reliability of the DNS servers I was using. The original configuration I used was the following:

$ dig mx grosseosterhues.com
[...]
;; ANSWER SECTION:
grosseosterhues.com.    120     IN      MX      10 mx.grosseosterhues.com.

and

$ dig mx.grosseosterhues.com
[...]
;; ANSWER SECTION:
mx.grosseosterhues.com. 119     IN      CNAME   mail.grosseosterhues.com.
mail.grosseosterhues.com. 120   IN      A       46.4.78.23

So my MX record was a CNAME pointing to an A resource record. No problem with that — I thought. Doing some web research I learned from RFC 2181, Section 10.3 that there is a certain regulation prohibiting an MX resource record from being an alias (CNAME):

10.3. MX and NS records: The domain name used as the value of a NS resource record, or part of the value of a MX resource record must not be an alias. Not only is the specification clear on this point, but using an alias in either of these positions neither works as well as might be hoped, nor well fulfills the ambition that may have led to this approach. This domain name must have as its value one or more address records. Currently those will be A records, however in the future other record types giving addressing information may be acceptable. It can also have other RRs, but never a CNAME RR.

I don’t know if some providers just adjusted their MTA to use more restrictive policies on sending mails to foreign mail servers of if I just never realized a problem. Anyway: after changing the MX record over to an A resource record everything works flawless (again).

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