Alun <auj@???> said, in message
20060512135215.1b7da9ca@???:
> It's fairly obvious that these are genuine outbound mail servers, but
> I'm very confused as to why doing it. We use greylisting, so for the
> latter three I guess it could be that they've tried the MX record,
> hit our greylisting and are now failing back to the A record. Would
> this be valid behaviour?
Should have looked at the RFC before asking that question:
5. Address Resolution and Mail Handling
[...]
The lookup first attempts to locate an MX record associated with the
name. If a CNAME record is found instead, the resulting name is
processed as if it were the initial name. If no MX records are found,
but an A RR is found, the A RR is treated as if it was associated with an
implicit MX RR, with a preference of 0, pointing to that host. If one or
more MX RRs are found for a given name, SMTP systems MUST NOT utilize any
A RRs associated with that name unless they are located using the MX RRs;
the "implicit MX" rule above applies only if there are no MX records
present. If MX records are present, but none of them are usable, this
situation MUST be reported as an error.
I read that as meaning that it's not correct behaviour to fall back to the A
record when the machines identified by the MX record defer.
Cheers,
Alun.
--
Alun Jones auj@???
Systems Support, (01970) 62 2494
Information Services,
University of Wales, Aberystwyth
