smtp;550 5.7.1 Unable to relay for

I have setup a new server at a ISP (in internet) with Windows 2003 Server R2 64 bit and have installed Micosoft Exchanger Server 2007 sp1

After the setup I can send emails from mynewserver.com to other servers and to my emial account, but when I reply to those messages, I get the following error:

Reporting-MTA: dns;bay0-omc3-s24.bay0.hotmail.com
Received-From-MTA: dns;BAY143-W20
Arrival-Date: Sat, 23 May 2009 18:04:01 -0700

Final-Recipient: rfc822;administrator@newserver.com.mx
Action: failed
Status: 5.7.1
Diagnostic-Code: smtp;550 5.7.1 Unable to relay for administrator@newserver.com.mx

Can you please help? It have setup a SMTP connector already.
HectorHugoGonzalezAsked:
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epochassetCommented:
You likely have a problem with your recipient policies.  It sounds like mynewserver.com is setup, but newserver.com.mx is not.  See http://www.computerperformance.co.uk/exchange2003/exchange2003_recipient_policy.htm.
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epochassetCommented:
Sorry that was for 2003.  In 2007 recipient policies are called email address policies.
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SurajCommented:
check if you have SMTP Service installed. if yes. disable it or uninstall it...
test the mail flow
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HectorHugoGonzalezAuthor Commented:
I reinstalled the Exchange Server and then added a Sender a the organization level and it worked.
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Windows Server 2003

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