You need to set it to an instance of the RDOAddressEntry object.
RDOAddressEntry object can be retrieved from another RDOMail object (RDOMail.Sender), RDOSession.AddressBook.ResolveName, or one of the address lists (RDOSession.AddressBook.AddressLists).
You might also want to set the RDOMail.SentOnBehalfOf property (it too needs an instance of the RDOAddressEntry object).