Although written as private correspondence to specific individuals, each of Symmachus' letters, when received, was read aloud to the members of the household and to friends; typically, the confidential bits of information or controversial views on public affairs would be conveyed by the letter carrier in private, oral conversation.
Salzman adds in a note: 'Symmachus mentions information being delivered orally by letter carriers often; see for example, Ep. 1.11, 1.46, 1.87.2, 1.90.1, 2.11, 2.21, 3.30, 4.44, 6.13, 8.31, 9.37. Ep. 6.18, notes that Symmachus gave oral information about a grain shortage to his letter carrier, so that the recipient of the letter will learn more by listening than by reading.' 
 J.F. Matthews, ‘The Letters of Symmachus’ in Latin Literature of the Fourth Century (ed. J.W. Binns; London: Routledge and Kegan Paul, 1974), 58–99.
 M. R. Salzman, ‘Travel and Communication in The Letters of Symmachus’, in Travel, Communication and Geography in Late Antiquity: Sacred and Profane (eds: L. Ellis & F. L. Kidner; Aldershot & Burlington VT: Ashgate, 2004), 81-94 (citation from p. 81).
 Salzman, ‘Travel and Communication in The Letters of Symmachus’, n. 1, pp. 81-82.