I don't know the application but when you google for Mimecast and user authentication I think the password is stored in the msw.s3db file.
One article says it stored Mimecast folder under the Program Files directory:
An other article says this file is stored in C:\Users\<username>\AppData\Roaming\Mimecast
Mimecast for Outlook: Integrated Windows Authen... | Mimecaster Central
Maybe you can have a look and add the appropriate folder location in the config file.