Contains the unique entry identifier of the message store where an object resides.
This property is used to open a message store with the IMAPISession::OpenMsgStore method. It is also used to open any object that is owned by the message store.
For a message store, this property is identical to the store's own PR_ENTRYID (PidTagEntryId) property. A client application can compare the two properties using the IMAPISession::CompareEntryIDs method.