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 PR_SENDER_NAME (PidTagSenderName) on the outbound
copy of the message, and leave it unset on the local copy.