By America, we mean America the continent, not America the nation. Mail service on the North American Continent predates the beginning of our nation by a century and a half. That's 150 years!
Almost 400 years ago, in 1637, the movers and shakers of the Colonies got tired of waiting for cell phone service. Land lines were hundreds of years away, and using a sailboat to carry messages from one port city to another was dependent on good weather. In New England, that was hit or miss, depending on the season. Another way had to be found to ferry messages between the social and financial hubs of Boston and New York. The cutting edge and most dependable transportation on the scene was the trusted four-legged equine. Soon, mail began to show up in Boston and New York with regularity and on time. Dispatches could be sent, love letters wafted between fiancés, and business transactions concluded without ever meeting face-to-face. It wasn't quite Facebook or video chatting, but it was 1637 state-of-the-art communication, nonetheless.
Just don't forget to feed the horses. If they had gone on strike, the whole shebang would have fallen apart faster than you could say cock robin!