Who was the greek historian Herodotus describing when he seaid thst neither snow rain heat or darkness would stop them?

Achaemenid Persia Postal service, dated back to 6th century BCE.

It is said that as many days as there are in the whole journey, so many are the men and horses that stand along the road, each horse and man at the interval of a day's journey; and these are stayed neither by snow nor rain nor heat nor darkness from accomplishing their appointed course with all speed.-Herodotus, Histories (8.98) (trans. A.D. Godley, 1924)