Ex parte Jackson was a United States Supreme Court case that dealt with the issue of whether the United States Postal Service had the authority to open and inspect mail without a warrant. The case arose when a package containing whiskey was sent through the mail and was opened and inspected by a postal inspector without a warrant. The sender of the package, Jackson, argued that the Postal Service had violated his Fourth Amendment rights by opening and inspecting the package without a warrant. The Supreme Court held that the Postal Service did not need a warrant to open and inspect mail. The Court reasoned that the Postal Service had a duty to protect the public from dangerous or illegal items sent through the mail, and that this duty outweighed the sender's Fourth Amendment rights. The Court also noted that the Postal Service had a long history of inspecting mail without a warrant, and that this practice had been accepted by the public. In conclusion, the Supreme Court held that the Postal Service did not need a warrant to open and inspect mail, and that the sender's Fourth Amendment rights were outweighed by the Postal Service's duty to protect the public from dangerous or illegal items sent through the mail.

In Ex Parte Jackson, the Supreme Court was tasked with determining whether a federal statute that prohibited sending obscene materials through the mail applied to newspapers. The majority opinion held that it did not, but Justice Field dissented from this ruling. He argued that Congress had intended for the law to apply broadly and thus should be interpreted as such. Furthermore, he noted that obscenity in any form is an affront to public decency and morality and should not be allowed by any means of communication or transportation within the United States. Therefore, he concluded that Congress had every right to prohibit its transmission via post office services regardless of what type of material it may contain.