On Apr 11 2009, Florian Weimer wrote:
The MX RR will be ignored. There will be an AAAA DNS request and a
fallback to the A RR for security.eu.debian.org. Newer versions of
sendmail and Postfix will treat that MX RR as a bad MX and reject the
message instead of retrying.
Exim also treats the record as a "no SMTP service here" indication. I
would even go so far to call this a de-facto standard (which just
hasn't been documented by the IETF).
However, it's maybe worth pointing out that Exim also provides support
for the alternative in which the absence of an MX record implies an
invalid mail domain. Specifically, the "mx_domains" option on a
dnslookup router specifies domains for which fallback to using an
address record should not occur (setting it to "*" would make that
apply universally). We have used this rule locally for domains under
cam.ac.uk for a very long time, which I am sure is why Philip Hazel
implemented the option in the first place.
Exim also provides the ability to use different retry rules in the
case when the target was found via an A or AAAA record, and these
are quite often used to give up (much) sooner on such deliveries.
--
Chris Thompson
Email: c...@cam.ac.uk
_______________________________________________
DNSOP mailing list
DNSOP@ietf.org
https://www.ietf.org/mailman/listinfo/dnsop
