What is "barid"

The barīd (, often translated as Postal Service) was the state-run courier service of the Umayyad and later Abbasid Caliphates. A major institution in the early Islamic state, the barid was not only responsible for the overland delivery of official correspondence throughout the empire, but it additionally functioned as a domestic intelligence agency, which informed the caliphs on events in the provinces and the activities of government officials.