Short answers, pulled from the story.
The first documented organized courier service began in 2400 BCE in Egypt. Pharaohs utilized couriers to disseminate decrees across their territory, establishing a precedent for state-controlled communication that would echo for millennia.
Darius the Great developed the first credible claim for a true postal system under the Achaemenid Empire. He reigned from 521 BCE and reorganized the Royal Road to facilitate rapid travel for Persian couriers stretching from Susa in the east to Sardis in the west.
By the end of Kublai Khan's rule, there were more than 1400 postal stations in China alone. These stations had at their disposal about 50,000 horses, 1,400 oxen, 6,700 mules, 400 carts, 6,000 boats, more than 200 dogs, and 1,150 sheep.
The true turning point for the common person arrived in the United Kingdom in 1840 when Sir Rowland Hill reformed the postal system. This reform led to the invention of the postage stamp known as the Penny Black, which democratized communication.
The world's first scheduled airmail post service took place in the United Kingdom on the 9th of September 1911. This service operated between the London suburbs of Hendon and Windsor, Berkshire.
The privacy of correspondence is guaranteed by the constitutions of Mexico, Colombia, Brazil and Venezuela. These rights are also alluded to in the European Convention on Human Rights and the Universal Declaration of Human Rights.