Mordecai wrote in the name of King Xerxes and sealed it with the royal signet ring. He sent the documents by mounted couriers riding on swift horses bred from the royal mares.
Jewish Publication Society of America Version (JPS1917) - Esther 8:10 >And they wrote in the name of king Ahasuerus, and sealed it with the king's ring, and sent letters by posts on horseback, riding on swift steeds that were used in the king's service, bred of the stud;