spam filter and MX records
Mark_Andrews at isc.org
Wed Feb 1 03:27:04 UTC 2006
> On 31 Jan 2006, at 15:20 PST, barber.greg at gmail.com wrote:
> > Well it's kind of a strange setup the mailer and the webmail interface
> > all reside on machine. Prior to the filter being put in place no MX
> > existed for this machine it was only an A record set to the email
> > domain so instead of being mail.xyz.123.edu or mx.xyz.123.edu it's A
> > record was just xyz.123.edu. I thought in a situtation like this the
> > change would be warranted in case the filters went offline foreign
> > mailers would sense that xyz.123.edu was down and requeue the message
> > instead of delivering straight to xyz.123.edu via the A record.
> I presume from your comments that xyz.123.edu is a subdomain of the
> parent 123.edu domain and that an A record had been defined to allow
> mail to be sent to "user.mailbox at xyz.123.edu". Further the A record
> that is associated with the domain name contains the IP address of
> the mail server.
> For robustness, the strategy employed by sendmail and Exchange's IMS
> is to query DNS for any record associated with xyz.123.edu. The DNS
> response will contain A, MX, NS, and any other associated record.
> If an MX record exists, sendmail and IMS will prefer to use this to
> deliver mail and will select the MX record with the lowest preference
> value. If the system with the lowest preference value is not
> reachable, they will attempt to deliver mail using an MX record with
> a higher preference value. If all of the systems identified in MX
> records are unreachable and there is an A record for the resource,
> they will attempt to deliver the mail using the A record.
Any MTA that falls back to A / AAAA records when MX records
are present is BROKEN.
A MTA is only supposed to fallback to a A / AAAA record when
there are *no* MX records.
> If you want all your mail relayed through a defined mail exchange
> system and never directly, you need to specify on one of your MX
> records a preference value of 0. This informs sendmail and IMS that
> you will only accept mail relayed through this system.
0 is not a special value.
> Most of the mail systems that don't understand MX records have been
> retired. There are a few still out there. To address this type of
> system, I would set the subdomain's A record to the address of your
> preferred or only mail exchange system: in your case, the mail
> filter system.
> As you can see, a part of the problem is understanding how systems
> make use of the information in DNS.
> Merton Campbell Crockett
> m.c.crockett at adelphia.net
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742 INTERNET: Mark_Andrews at isc.org
More information about the bind-users