The entry identifiers of certain special folders (such as the IPM Subtree, the Outbox, and the Sent Items folder) are stored as properties on the information store objects that contain them.
To open a special folder
Open the private, personal, or public information store. This returns an LPMDB pointer to the open information store. See Opening an Information Store.
Open the folder within the information store by calling the MAPIIMsgStore::OpenEntry method. In this function call, pass the entry identifier of the folder in the lpEntryID parameter. For a list of entry identifiers, see Entry Identifiers of Special Folders and Expressing Paths to Folders.