'Service' in the Bible

He wrote [a decree] in the name of King Ahasuerus, and sealed it with the king’s ring, and sent letters by couriers on horseback, riding on the royal [mail] relay horses, the offspring of the racing mares.

So the couriers, who were mounted on the royal relay horses, left quickly, urged on by the king’s command; and the decree was issued at the citadel in Susa [the capital].