One email user receives a 5.1.2 'Invalid destination email system' when attempting to reply to or send to a particular address.


I have a weird problem going in my network.  We have an Exchange 2003 SP2 mail server running on a server 2003 platform.  We have a mix of Outlook 2007 and Outlook 2010 clients.

One of our users (on a Outlook 2007 client) is receiving the following message when attempting to reply to or send a new email to a specific domain:

Your message did not reach some or all of the intended recipients.

      Subject:      FW: proof of delivery as requested for PO S82FSAN00001
      Sent:      4/19/2011 4:52 PM

The following recipient(s) cannot be reached:

      '' on 4/19/2011 4:52 PM
            The message could not be delivered because the recipient's destination email system is unknown or invalid. Please check the address and try again, or contact your system administrator to verify connectivity to the email system of the recipient.
            < #5.1.2>

The funny thing is, no other users in our network, Outlook 2007 or 2010 clients, are affected.

To me, that screams something on the client level, but for the life of me I can't figure out what.

Does anyone have any ideas?  
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Check the user's Contact folder for the contact.

Also try deleting it out of the users nickname cache by starting to type the address in the "To" box then pressing delete key on the suggestion when it comes up.

You can also try sending to that address from the user logged into OWA. If it works there then it definitely is an outlook issue. Then you can try a new Outlook profile (leave the original one alone) and test again
The message you get in the email is sent by (maybe your exchange server) and it means that the destination email/domain doesn't exist or is incorrect. If you know this email ( is a valid email, you can try to send a email from another email system, like hotmail, gmail, etc. And verify that the destination email exist.

In some ocassions spammer send email from any server with senders that doesn't exist.

meelnahAuthor Commented:
I understand what you are saying, but other users in our domain (using the same mail server) can send to the email address... only the one user is affected.
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Have you review the SMTP Virtual server permissions, or maybe could be a configuration for SMTP Send Connector for that user. Maybe you can trace the problem with the tool Message Tracking Center in the Folder Tools in Exchange System Manager.

Have you tryed to enable the log for the smtp server. Here is some info

Another doc to log to trace smtp problems.
meelnahAuthor Commented:
Was a problem with the user's Outlook client.  Web mail went through fine.  Once I deleted her contact for that user, and also wiped out the nk2 file, emails are now flowing correctly... weird, but true!

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