The Mongol Empire's postal system covered 5,000 miles

Genghis Khan's yam (relay post) system, built from the 1220s, was the ancient world's most sophisticated communications network. Relay stations were placed every 25–30 miles across the empire, with fresh horses and riders at each. Official messengers could cover 200 miles per day — a pace not matched by any postal system until the 19th-century Pony Express. The system required maintaining roughly 50,000 horses across the routes. Marco Polo marvelled at it in the 1270s, calling it one of the greatest achievements he encountered in Asia.

Source: Man, Genghis Khan: Life, Death and Resurrection (2004)