Pass windows credentials to Exchange 2007 web service VB.NET

Question

I am trying to interop to Exchange 2007 via its web-services and have the following issue;

Authentication mode = “windows” in the web.config….

Why isn’t the default credential passing the current user to the exchange webservice (EWS)?

    Dim esb As New mailserver.ExchangeServiceBinding()
    'Provide the NetworkCredential
    esb.Credentials = CredentialCache.DefaultCredentials


    'Provide the URL
    esb.Url = "https://server.domain.local/EWS/exchange.asmx"

It returns no mailbox found when I try and add a calendar item.I’m pretty sure it’s not passing the credentials.

"When making a request as an account that does not have a mailbox, you must specify the mailbox primary SMTP address for any distinguished folder Ids."

Yet when I hard code my credentials in, it works fine.