551 This is not a relay host

Ive setup an SMTP server on a windows 2000 pro PC, everything seems to be working ok, apart from when a user tries to email 1 particular user I get:-
     551 This is not a relay host

The recipient is on an external address, so it does have to go out, but seems to work ok, apart from this 1 user

The users email is apparently correct.

Im completly confused!

Who is Participating?
WadskiConnect With a Mentor IT DirectorCommented:
What is happening is that the SMTP Engine has been configured to send mail but is preventing itself being an open relay.

The reason you are getting the message is that the message does not contain a valid address within the Domains section in IIS.

If you go in IIS to the Default SMTP Virtual Server you will see a + and in there will be Domains and Current Sessions.
Within the domains you need to add any domain that your company will send FROM.  

If you right click in the domain name section and click New > Domain then add an Alias for your domains you use.
tonelm54Author Commented:
The exact error message is:-
Diagnostic-Code: smtp;551 This is not a relay host - mail must be to or from host domain.
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