On Apr 11, 2009, at 4:25 AM, 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).
It would incorrect to describe MX records targeting the root as being
a widely adopted standard to signal "No SMTP Service".
In the past, Paul Vixie raised concerns about even using root targets
within SRV records, which has always been defined as a means to signal
no service. He said that his experience at the root had shown
programmers should not be trusted to properly recognize root domains
within SRV records. In the case of SMTP, there was never a standard
to properly ignore root targets. A signaling scheme that shifts the
signaling of no SMTP service responses to the root may prove
detrimental.
-Doug
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