CDO and Exchange

I'm writing an application where I update an exchange server with data using CDO 1.21. My problem is that when a field on an user object doen't exist and I try to add it to the fields collection then I get AccessDenied. Our exchange administrator have changed my rights but it still don't seem to work.

Does anybody know how to do this right?

You must pass a value to every field just not the one's that you want to update.. that is why your getting the error so if its a null value pass a 0 or a space