"It’s a question fit for Benjamin Franklin.
Prior to the American Revolution, Franklin had been the postmaster for the British Crown, establishing postal delivery routes throughout the colonies. In the early days, it was only official government communications that passed through the post, and it was “sealed against inspection”.
Later, when the mail could be used by citizens, carriers would regularly read others’ mail along their long routes for entertainment.
Franklin, eager to maintain the sanctity of the mail in a time of political upheaval, developed a set of regulations and affixed locks to postal carriers’ saddle bags. Franklin’s early regulations became part of the basis for privacy law, as did the Fourth Amendment rule about unreasonable searches, which the Framers certainly intended to cover postal mail."
source: https://slate.com/technology/2013/06/nsa-surveillance-why-th...