sender. It must be set by the outgoing transport provider, which should
never propagate any previously existing values.
If no transport provider has supplied any sender address properties, the
MAPI spooler attempts to fill them in by calling the
IMAPISession::QueryIdentity method for an entry identifier. If no entry
identifiers have been provided, the MAPI spooler stores "Unknown" in all
the sender address properties of type PT_TSTRING.