Douglas Harper's Etymology Dictionary
postillon
1590s, from Middle French postillon (1530s), from Italian postiglione "forerunner, guide," especially for one carrying mail on horseback, from posta "mail" (see post (n.3)) + compound suffix from Latin -ilio.
Usage examples of "postillon".
III VIENNA On the 29th November, Casanova wrote from Frankfort that a drunken postillon had upset him and in the fall he had dislocated his left shoulder, but that a good bone-setter had restored it to place.