Removing Anonymous Access on Exchange for postfix relay (inbound)

Hi good afternoon. I'm trying to make the Posftix authenticate to the Exchange to deliver mail to local accounts. In the SMTP Virtual server Exchange 2003 i disable anonymous access and configure postfix to authenticate with an account that exists in the Active directory. The problem is that now in the postfix log of when you want to try to deliver a mail coming from an external domain throws me:

said: 454 5.7.3 Client does not have permission to Send As This sender.

I missed to configure something? The idea is not to allow anonymous access but the SMTP server.
Someone had to do something similar?

Thank you very much.