For most of the nation’s history, the First Amendment was held to not cover anything judged to be obscene, and of course defining what is obscene occupied a great deal of judicial time and effort. The frustrations were notably summed up in Justice Potter Stewart’s memorable 1964 phrase, concurring in Jacobellis v. Ohio, 378 U.S. 184, that “I know it when I see it.” For those of us who studied law in the 20th century, it’s been interesting to see this entire thicket of legal reasoning just mowed down by the Internet.

As this 1960 law review article explains, the Supreme Court in 1878, in Ex parte Jackson found that inherent in Congress’s constitutional power to establish post offices was the power to decide what was mailable:

What should be mailable has varied at different times, changing with the facility of transportation over the post roads. At one time, only letters, newspapers, magazines, pamphlets, and other printed matter, not exceeding eight ounces in weight, were carried; afterwards, books were added to the list; and now small packages of merchandise, not exceeding a prescribed weight, as well as books and printed matter of all kinds, are transported in the mail. The power possessed by Congress embraces the regulation of the entire postal system of the country. The right to designate what shall be carried necessarily involves the right to determine what shall be excluded.

The disputed material in Jackson was a solicitation to take part in a lottery, but a 19th century Court had no trouble seeing that as within Congressional power:

In excluding various articles from the mail, the object of Congress has not been to interfere with the freedom of the press, or with any other rights of the people, but to refuse its facilities for the distribution of matter deemed injurious to the public morals.

The Court, and courts following Ex parte Jackson, had little difficulty finding that Congress could therefore exclude obscene materials, which have never enjoyed First Amendment protection. With regard to the Fourth Amendment, the same case continues:

a distinction is to be made between different kinds of mail matter -- between what is intended to be kept free from inspection, such as letters, and sealed packages subject to letter postage, and what is open to inspection, such as newspapers, magazines, pamphlets, and other printed matter purposely left in a condition to be examined. 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.

Thus, you can write as lasciviously as you like to your boyfriend in sealed personal correspondence—but printed matter entered into the mail is subject to regulation regarding its contents.