Object API can be run as part of an NT service, with the following exceptions:
Call the MultiLogin method instead of the normal Login method to access accounts. An NT service cannot display any User Interface (dialogs, etc.) that the normal Login method can display (to prompt for password or in case of a login error). MultiLogin does not display any dialog boxes. It simply returns login errors to the caller.
If you must call the Login method, make sure the login will be successful without any other user interaction. An NT service cannot display User Interface. If the Login method has errors or needs a password, it will try to display Dialog Boxes, which violates this rule.
Any calls to MultiLogin or Login must pass in the "/ntservice" parameter as part of the login command line. This is the only way that Object API knows it is running as part of an NT service. GroupWise needs this parameter to properly login to the MAPI address book providers.