How did the ancient Persian Empire create the world's first postal system?

The answer

The Achaemenid Persian Empire (550–330 BCE) established the Angarium, a relay postal system spanning the 2,700-kilometer Royal Road from Susa to Sardis. Mounted couriers passed messages between 111 relay stations, completing the journey in roughly 7 days - a trip that took ordinary travelers 90 days. Herodotus famously wrote that 'Neither snow nor rain nor heat nor gloom of night' could stop them - a phrase later adopted as the unofficial motto of the U.S. Postal Service.