The Comstock Act was passed by Congress in 1873. Its stated purpose was "Suppression of Trade in, and Circulation of, Obscene Literature and Articles of Immoral Use." The Comstock Act made it illegal to use the U.S. Postal Service to send sex related material including, but not limited to, birth control, sex toys, erotica, abortifacients, letters that discussed sexual matters, and any information about the things on the preceding list.
The Comstock Act involved a very broad interpretation of what can be considered "obscene." It remained in place until 1957, with local governments keeping their own applications of the law in place for an additional eight years.