Why do I get the error message "The SendUsing configuration value is invalid"
This problem occurs because read access to the Internet Information Services (IIS) metabase and the Microsoft Active Directory directory service has been removed for the Everyone group. This access has been removed because of a security modification in Exchange 2000 Server SP3. CDOEX, CDOSYS, and System.Web.Mail must have access to the IIS metabase to access information about the location of the pickup directory path. This behavior occurs when you use the Sendusingpickup method and if this information is not specified in the application code. Because access is restricted, the non-administrative user under whose security context the application is running cannot read this information from the IIS metabase and Active Directory.
For more information refer Read Access to the Everyone Group Is Removed After You Install Exchange 2000 SP3