Outlook account not sending external email

A particular user on Exchange server 2003 gets this error when sending external email:

You do not have permission to send to this recipient.
server name.domain.local #5.7.1 smtp;554 5.7.1
[email protected] unknown.

Have compared the settings to a working one, and all look OK. We also tried recreating the user profile and email address but it still doesn't work.

Any suggestions?
 
As you got the error back from the server, there is nothing wrong with the client or Outlook. It is probably some setting on the server side, blocking this specific computer, user or recipient.
 
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