Ran across this interesting NYT article from 1908. After President McKinley was assassinated by an anarchist, Teddy Roosevelt demanded action against anarchist publications being sent through the postal service. And yet he clarifies this does not apply to normal mail - "sealed documents" - explaining the government is "expressly forbidden to ascertain, what the purport of such messages may be":

The greater portion of his opinion is devoted to the question of whether, in the absence of any legislation by Congress, the Postmaster General has the right to exclude such publications. On this point his conclusion is: "The Postmaster General will be justified in excluding from the mails any issue of any periodical, otherwise entitled to the privilege of second-class mail matter, which shall contain any article constituting seditious libel, and counseling such crimes as murder, arson, riot, and treason." The Attorney General makes a clear distinction with reference to the authority of postal officials over sealed and unsealed mail matter. In conveying letters and newspapers to persons to whom they are directed, he says the United States "undertakes the business of a messenger." He adds: "In so far as it conveys sealed documents, its agents not only are not bound to know, but are expressly forbidden to ascertain, what the purport of such messages may be; therefore, neither the Government nor its officers can be held either legally or morally responsible for the nature of the letters to which they thus, in intentional ignorance, afford transportation."

https://www.nytimes.com/1908/04/10/archives/roosevelt-demand...