This property is an example of the address properties for the messaging user being represented by the sender. When a client application sends a message on behalf of another client, it should set all the represented sender properties to the values for that client. A messaging user sending on its own behalf typically leaves the represented sender properties unset.
The outgoing transport provider must always leave this property unchanged if it has been set by the sending client. If it is unset, the transport provider should set it to the PR_SENDER_SMTP_ADDRESS (PidTagSenderSmtpAddress) property on the outbound copy of the message, and leave it unset on the local copy.