Post Office Act (1872)
The Post Office Act (17 Stat. 283, enacted June 8, 1872) formally incorporated the United States Post Office Department into the United States Cabinet. It is also notable for §148 which made it illegal to send any obscene or disloyal materials through the mail, to be the foundation of the later Comstock Act of 1873.
It was preceded by the Postal Service Act of 1792 which created the Post Office Department, and succeeded by the Postal Reorganization Act of 1970 which abolished the Post Office Department and replaced it with the current United States Postal Service (USPS).