In 1775, the Second Continental Congress established the first organized mail service in America, the Constitutional Post. Prior to this service people relied on friends, merchants, or private messengers to carry letters. This system was often unreliable, particularly because the British postal inspectors intercepted confidential messages.
The first Postmaster General was Benjamin Franklin, who established many of the conventions we are accustomed to today, such as a standardized rate chart based on weight and distance. Before stamps were invented in 1847, the sender could pay the postage in advance or leave it for the recipient to pay upon arrival.
In the past two centuries, the Postal Service has grown and changed dramatically, but its mission of promoting free and open communication remains the same.