EX 5.5 SP4 on NT SP6a

Messages sent to users at certain domain in Europe (Italy) from our Exchange
mailbox bounce back undelivered as "User unknown":
However when they are sent from a Yahoo account, they get delivered without
a problem.
I verified connectivity to recipient's mail server by pinging and telnetting
to port 25 successfully.
I even specified the recipient's mail server IP address instead of relying
on DNS, but the problem still persisted.
I turned on the SMTP Protocol Logging on IMC. Here's the result:


++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
2/5/03 11:49:31 AM : A connection to 195.120.145.5 was established.
2/5/03 11:49:31 AM : <<< 220 addon.it ESMTP MDaemon 6.0.3; Wed, 05 Feb 2003
17:47:16 +0100
2/5/03 11:49:31 AM : >>> EHLO s-mail.visionics.com

2/5/03 11:49:32 AM : <<< 250-addon.it Hello s-mail.visionics.com, pleased to
meet you

2/5/03 11:49:32 AM : <<< 250-ETRN
250-AUTH LOGIN CRAM-MD5
250-8BITMIME
250 SIZE 0

2/5/03 11:49:32 AM : >>> MAIL FROM:<[EMAIL PROTECTED]> SIZE=3830

2/5/03 11:49:32 AM : <<< 250 <[EMAIL PROTECTED]>, Sender ok
2/5/03 11:49:32 AM : >>> RCPT TO:<[EMAIL PROTECTED]>

2/5/03 11:49:33 AM : <<< 550 <[EMAIL PROTECTED]>, Recipient unknown
2/5/03 11:49:33 AM : >>> QUIT
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Also we have been having similar problems with other european recipients.
Any ideas what could be causing this? 


Thanks

--Alex Alborzfard
Network Errand Boy
MCSE, CCNA, CCDA, CCISP

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