[Exim] Sender address does not resolve?

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Hi Members,



Hello all. I am running Exim 4 together with Mailscanner and ClamAV.
Recently I am facing a problem with a valid email address (user@???)
sending to the recipient recipient@??? . The email could not be sent
out because the recipient's server claims that user@??? is not being
able to resolve. The problem is that this same user@??? is able to
send messages to other accounts. The following are the details:



Connecting to earth.gems2.com.sg [160.96.65.13]:25 ... connected
SMTP<< 220 earth.gems2.com.sg ESMTP Singapore Government E-Mail System;
Fri, 9 Jan 2004 01:31:21 +0800 (SGT)
SMTP>> EHLO my.hostname.com

  SMTP<< 250-earth.gems2.gov.sg Hello my.hostname.com [123.12.123.123],
pleased to meet you
         250-ENHANCEDSTATUSCODES
         250-PIPELINING
         250-8BITMIME
         250-SIZE 30720000
         250-DSN
         250-ETRN
         250-DELIVERBY
         250 HELP

SMTP>> MAIL FROM: SIZE=15343
SMTP>> RCPT TO:
SMTP>> DATA
SMTP<< 451 4.1.8 Domain of sender address user@??? does not resolve
SMTP<< 503 5.0.0 Need MAIL before RCPT
SMTP<< 503 5.0.0 Need MAIL command
SMTP>> QUIT
LOG: MAIN

== recipient@??? R=lookuphost T=remote_smtp defer (0): SMTP error
from remote mailer after MAIL FROM: SIZE=15343: host earth.gems2.com.sg
[678.67.67.67]: 451 4.1.8 Domain of sender address user@??? does not
resolve



May I know what is the problem? I tried DNSreport on both user@???
and recipient@??? and the results are fine. Is there a way to solve
the problem?



Thank you very much in advance.



Best regards,

CH WONG

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