The IMAPIMessageSite::GetStore method returns the message store containing the current message, if such a store exists. This method will return NULL in the ppStore parameter for embedded messages, which are stored within another message rather than directly in a message store.
See IMAPIMessageSite : IUnknown.
HRESULT GetStore(
LPMDB FAR * ppStore
);
For a list of interfaces related to form servers, see MAPI Form Interfaces.