The umbrella collects two unrelated senses of the word: dispatches carried by royal couriers, and a piece of body armor.
Royal Couriers
In the Persian court of Esther, official decrees travel by mounted post-runners. Haman's order goes out under the king's authority: "And letters were sent by posts into all the king's provinces, to destroy, to slay, and to cause to perish, all Jews, both young and old, little children and women, in one day, even on the thirteenth [day] of the twelfth month, which is the month Adar, and to take the spoil of them for a prey" (Es 3:13). Mordecai's countermanding edict goes out the same way, with extra emphasis on the speed of the dispatches: "And he 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" (Es 8:10).
A Coat of Mail
The other sense of the word is body armor. Goliath's panoply is described piece by piece, and the coat of mail is the central item: "And he had a helmet of bronze on his head, and he was clad with a coat of mail; and the weight of the coat was five thousand shekels of bronze" (1Sa 17:5).