spam filter and MX records

Merton Campbell Crockett m.c.crockett at
Wed Feb 1 05:04:59 UTC 2006

On 31 Jan 2006, at 19:27 PST, Mark Andrews wrote:

>> On 31 Jan 2006, at 15:20 PST, barber.greg at 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 or it's A
>>> record was just I thought in a situtation like this the
>>> change would be warranted in case the filters went offline foreign
>>> mailers would sense that was down and requeue the  
>>> message
>>> instead of delivering straight to via the A record.
>> I presume from your comments that is a subdomain of the
>> parent domain and that an A record had been defined to allow
>> mail to be sent to "user.mailbox at".  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  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.

Yes, RFC 2821 does make the statement that if there are any MX  
records present the A record is to be ignored.

There are also broken mail systems.  They will deliver mail to the A  
record.  If you don't want that to happen, observe the "prudent man"  
principle.  Provide an A record to the system that you want them to  
use for relaying mail.

>> 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.

See RFC 2821 regarding the implicit MX record.  If there is only an A  
record, it is to be treated as if it were an MX record with a  
preference of 0.  This seems somewhat special to me.

>> 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
> --
> Mark Andrews, ISC
> 1 Seymour St., Dundas Valley, NSW 2117, Australia
> PHONE: +61 2 9871 4742                 INTERNET: Mark_Andrews at

Merton Campbell Crockett
m.c.crockett at

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