Unable to email out, bouncing back.

I have having trouble email out to one particular person.
( I do not know if it is specif to that user or all users on that domain)
All other email to other domains is fine.

The error that bounces back is:
Your message did not reach some or all of the intended recipients.

      Subject:      RE: 07 items - Adria.xls
      Sent:      11/10/2008 10:59 AM

The following recipient(s) could not be reached:

      xxx@xxxx.com on 11/10/2008 11:00 AM
            There was a SMTP communication problem with the recipient's email server.  Please contact your system administrator.
            <webmail.xxx.com #5.5.0 smtp;553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)>
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* Would you like to check the domain of the user whom you are trying to send emails whether it has been blocked or marked as an open relay for spammers


* You could even have a look for your domain too for conformity

Good Luck (^_^)

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