Default MDN Construction

A default MDN is created not as a result of a subject message, but by just calling the BuildMDN method.  The properties in the default MDN show what headers and body are generated at a bare minimum -- when no subject message has been loaded.  This is useful for when you need to create an MDN from scratch, and all that is required after creating the default MDN is just overriding the existing values with the correct ones.  However, when a message has been loaded, the MDN generated when calling the BuildMDN method will depend on the values of the loaded message.  The default MDN consists of: