The Earliest Surviving Letter Written in Greek

The Greek historian Philochorus of Athens (Φιλόχορος) is credited with the invention of the "typically scientific letter and the polemical pamphlet." A letter by Philochorus written on lead plates, μολυβδινη επιστολη, survived from the early fourth century CE and was preserved in the Kgl. Museen, Berlin. 

Adolf Wilhelm, "Der älteste grieschische Brief," Jahreshefte des oesterreichen Archäologischen Institutes in Wien, 7 (1904) 94-105 (with illustrations of the original letter written on lead).

Jenö Platthy, Sources on the Earliest Greek Libraries with the Testimonia (1968) 29.