Halliday & Matthiessen (2014: 306):
The Receiver is the one to whom the saying is directed … The Receiver may be Subject in a clause that is ‘receptive’ … The Receiver is realised by a nominal group typically denoting a conscious being (a potential speaker), a collective or an institution; the nominal group either occurs on its own or is marked by a preposition — almost always to but sometimes of. The range of realisational possibilities depends on the lexical verb of the verbal group realising the Process …