Ex parte Jackson, 96 U.S. 727 (1877) (full-text).
Factual Background Edit
U.S. Supreme Court Proceedings Edit
In discussing the nature of protecting the postal system, the Court observed that:
|“||Letters and sealed packages of this kind in the mail are as fully guarded from examination and inspection, except as to their outward form and weight, as if they were retained by the parties forwarding them in their own domiciles. The constitutional guaranty of the right of the people to be secure in their papers against unreasonable searches and seizures extends to their papers, thus closed against inspection, wherever they may be. Whilst in the mail, they can only be opened and examined under like warrant, issued upon similar oath or affirmation, particularly describing the thing to be seized, as is required when papers are subjected to search in one's own household. No law of Congress can place in the hands of officials connected with the postal service any authority to invade the secrecy of letters and such sealed packages in the mail; and all regulations adopted as to mail matter of this kind must be in subordination to the great principle embodied in the fourth amendment of the Constitution.||”|
The Court relied on a content/non-content distinction to find that the inside of the letter (the content) was protected, while the routing information on the outside (non-content) was not subject to similar Fourth Amendment restrictions.