In “Wiretapping before the Wires: The Post Office and the Birth of Communications Privacy,” Anuj C. Desai of the University of Wisconsin Law School explains that the extension of the Fourth Amendment to cover postal mail, and then later to telephones, is based not so much on the inherently Constitutional nature of opening mail, but instead on the increasingly firm belief in the sanctity of the mail as expressed by Congress, legislators, and the public.

She writes, “The general process, of which Ex parte Jackson is an example, can be described briefly in four steps: (1) Congress passes a statute; (2) the statutory provision gives an institution certain attributes; (3) over time, social practice embeds those attributes into the institution; and (4) the courts then take those attributes and write them into constitutional law.”

In other words, the 1878 case of Ex parte Jackson was not based on originalism. It does not appear that the Founders really thought of postal mail–at least, the way it ran at the time–as being protected by the Fourth Amendment, although they did inherit the British sense of the importance of protecting its confidentiality. Desai notes that Professor Telford Taylor explained this when he noted, “It is quite impossible to spell out an original understanding that the mail, or any future means of general communication, were to fall within the ‘persons, houses, papers, and effects’ protected by the fourth amendment.”

To summarize Desai’s argument in more detail: pre-Revolutionary War policies and procedures generally protected the confidentiality of postal mail, but did so in a spotty enough fashion in practice that American colonists saw the importance of greater protections against government spying. Nonetheless, postal mail was not explicitly placed under Fourth Amendment protections, and instead statutory law (and, arguably, custom) protected the confidentiality of the mails.

According to Desai, these statutory protections became so ingrained that, when the federal government finally attempted in the 1870s to routinely censor letters (in the guise of enforcing a prohibition against lotteries), the Supreme Court felt that the importance of protecting the postal system from government search and seizure had finally risen to the level of requiring Constitutional protections. The Supreme Court then enshrined what had been statute and custom into the firmer bedrock of the Constitution.