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Exchange 2003 server; Getting 503 reject messages intermittently from only one recipient

Nyn11Tech asked
Last Modified: 2013-11-30
We receive these mail rejects periodically from only one recipient.  All other mail works fine and has been fine for 4 years.  When we receive this reject, subsequent mail to this recipient works for one to several days, then we get a reject again.  It may actually reset daily - we are a 24-hour operation so it is difficULt to tell.  We will get these rejects for mail sent to any user in the recipient's domain.

The reject message we get is:

The following recipient(s) cannot be reached:

      Sample User on 11/3/2008 9:34 AM
            There was a SMTP communication problem with the recipient's email server.  Please contact your system administrator.
            <email.int.ourdomain.org #5.5.0 smtp;503 This mail server requires authentication when attempting to send to a non-local e-mail address. Please check your mail client settings or contact your administrator to verify that the domain or address is defined for this server.>

We are running MS Server 2003 running Exchange Server 2003 SP2.

The recipient is using a Barracuda device and says when this happens he does not see the mail even hitting his filter.

I can successfully perform an NSLookup internally and it resolves to the recipient correctly.

Any ideas would be GREATLY appreciated.
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