The IMAPIMessageSite::GetFolder method returns the folder in which the current message was created or opened, if such a folder exists. This method returns NULL in the ppFolder parameter for embedded messages, which are not stored directly in a folder.
See IMAPIMessageSite : IUnknown.
HRESULT GetFolder(
LPMAPIFOLDER FAR * ppFolder
);
For a list of interfaces related to form servers, see MAPI Form Interfaces.