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Subject: Re: Incessant Bounces
From: Rask Ingemann Lambertsen (rask-postfixkampsax.k-net.dk)
Date: Wed Aug 23 2000 - 15:03:52 CDT

Den 23-Aug-00 19:28:06 skrev Brad Knowles fĝlgende om "Re: Incessant Bounces":
>At 6:49 PM +0200 2000/8/23, Rask Ingemann Lambertsen wrote:

>> $ host -t MX netscap.com
>> netscap.com mail is handled (pri=0) by mail.x1netscap.com
>> $ host -t A mail.x1netscap.com
>> Host not found.

> Yup. So the next level of checks that will have to be
>implemented on MTAs is to ensure that the target(s) of one or more MX
>records actually resolves into one or more IP addresses, and we may
>even have to take yet another step and correlate the IP addresses of
>the target(s) of the MX record(s) back to the IP address of the
>sender.

   I'm not sure what you are trying to say in the last part, but some
domains are set up in such a way that all MX'es (usually just one) point to
other peoples mail servers, which will of course reject the messages with
"Relaying denied" type messages. Unfortunately (?), the DNS does not have
any reverse MX information.

> If you could be truly sure that the NXDOMAIN response you got
>wasn't caused by problems in your local resolver or your local
>caching nameserver, then I'd agree with you.

> But you can't, so I don't.

   You can ask the authoritative servers directly before concluding that
the lookup failed. If the authoritative servers say it isn't there, then it
isn't there.

Regards,

/ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻTŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ\
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