authenticating to exchange 2010 smarthost?

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http://serverfault.com – I have a postfix mail server that should relay all outgoing mail to an Exchange 2010 server (the Exchange box is my smarthost). I have administrator access to the Exchange 2010 system, but I'm not very familiar with it. How should I set up authentication on the Exchange 2010 system? I guess I could add a standard user with a mailbox on the Exchange box, then configure my postfix box to log in to port 587 to relay mail. That option doesn't feel right -- it seems like there should be way to do server to server authentication, not just client to server authentication. Is there? If so, how w (HowTos)