error 572 "host name is unknown"

[Brian Gibson]

Got a strange one here. An outside sender's message is getting through
our frontline Barracuda anti spam device but our CGPro server (version
5.0.5) is rejecting the message with a 572 error code. The error code
then has the person's email address and it states "host name is
unknown".

I did find something interesting in the logs for their domain name. I
think what is happening is their main MX server is getting bogged down
and we can't reach it so it tries to connect to the backup but there is
no DNS "A" record for it.

18:58:27.88 3 SMTP-66317(leethies.com) failed to connect to
mx1.leethies.com [130.105.11.50:25]. Error Code=no response
18:58:59.84 2 SMTP-66317(leethies.com) [6473860] sent to
[130.105.12.3:25], got:250 SAA23024 Message accepted for delivery
18:58:59.84 2 DEQUEUER [6473860] SMTP(leethies.com)ga...@leethies.com
relayed: relayed via mx1.leethies.com
19:32:03.70 3 DNR-25745(mx2.leethies.com) A:host name is unknown
19:32:03.70 1 SMTPI-91987(mx.wheatonma.edu) Return-Path
'ga...@leethies.com' rejected: host name is unknown

Brian Gibson
Systems Administrator
Wheaton College
http://ideaman.edublogs.org

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[Brian Gibson]

I found the issue. The sender had equal MX weights in DNS but did not
have an A record for one them. Every other time our server did an MX
lookup to make sure the sender's domain was valid it would puke because
of the missing "A" record so it would reject the message.

Brian Gibson
Systems Administrator
Wheaton College
http://ideaman.edublogs.org

[Jeff Wark]

That is the exact same problem that I encountered last week with hockeycanada.ca. Three
equally weighted MX records and two of the hosts did not have A records. Interesting new
affliction some DNS servers seem to have now. ;)

Have a good one.

Jeff Wark
TBayTel Internet Operations