While Xerxes was doing thus, he sent a messenger to the Persians, to announce the calamity which had come upon them. Now there is nothing mortal which accomplishes a journey with more speed than these messengers, so skilfully has this been invented by the Persians: for they say that according to the number of the days of which the entire journey consists, so many horses and men are set at intervals, each man and horse appointed for a day’s journey. These neither snow nor rain nor heat nor darkness of night prevents from accomplishing each one the task proposed to him, with the very utmost speed. The first then rides and delivers the message with which he is charged to the second, and the second to the third; and after that it goes through them handed from one to the other, as in the torch-race among the Hellenes, which they perform for Hephaistos. This kind of running of their horses the Persians call angareion.
“The riders were exclusively in the service of the Great King and the network allowed for messages to be transported from Susa to Sardis (2699 km) in nine days; the journey took ninety days on foot. (Kia, Mehrdad (2016). The Persian Empire: A Historical Encyclopedia. Santa Barbara: ABC-CLIO. p. 127.)(https://en.wikipedia.org/wiki/Angarium)