I explain the history of postal surveillance and show that the principle of communications privacy derives not from the Fourth Amendment or even from the Constitution at all. Rather, it comes from early postal policymakers who put that principle into postal ordinances and statutes in the late eighteenth century. Over time, the principle of communications privacy became embedded into the postal network by both law and custom. It was only then that the Court incorporated it into the Fourth Amendment in the 1878 case Ex parte Jackson, which in turn served as one of the bases of Justice Brandeis's Olmstead dissent. So, if today we see the principle of communications privacy as fundamental to the Fourth Amendment, we have postal policymakers to thank, for it was through the post office, not the Constitution or the Bill of Rights, that early Americans first established that principle.