The PR_SENDER_EMAIL_ADDRESS property is one of the address properties for the message 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.
For more information on the address properties, see Base Address Properties.