Mu'awiyah introduced postal service, Abd al-Malik extended it throughout his empire, and Walid made full use of it. The Umayyad Caliph Abd al-Malik developed a regular postal service. Umar bin Abdul-Aziz developed it further by building caravanserais at stages along the Khurasan highway. Relays of horses were used for the conveyance of dispatches between the caliph and his agents and officials posted in the provinces. The main highways were divided into stages of 12 miles (19 km) each and each stage had horses, donkeys or camels ready to carry the post. Primarily the service met the needs of Government officials, but travellers and their important dispatches were also benefitted by the system. The postal carriages were also used for the swift transport of troops. They were able to carry fifty to a hundred men at a time. Under Governor Yusuf bin Umar, the postal department of Iraq cost 4,000,000 dirhams a year.
Try to answer this question if possible (otherwise reply "unanswerable"): Who stopped the use of horses in the postal service?
unanswerable