Using Integrated Authentication with EWS (Exchange Web Services)

I am in the business of creating a company intranet. One of the requests is to do some integration with Exchange Server 2007.

In my tests, I can fully authenticate the EWS with NetworkCredential while I walk past my username and password. On our intranet, I would like to transfer the power of attorney of the registered user. We will only use Integrated Windows Authentication.

I can't get this to work - setting credentials on the ExchangeServiceBinding for CredentialCache.DefaultCredentials doesn't work.

.net and IIS 6 from Exchange 2007 Web Server on Windows 2003 Server

Any ideas? Thank.