Hope Williard (Utrecht University) wrote ‘Letter Carriers and the Scale of Communication in the Epistolae of Sidonius Apollinaris’ for Kristina Sessa and Kevin Uhalde (eds), Scale and the Study of Late Antiquity, Munera 53, Bari: Edipuglia, 2023, 153-67.
Catalogue volume here
Preprint version article on Academia here