Police must obtain a warrant before accessing emails stored by internet service providers, a federal appeals court has ruled in a landmark decision that also struck down part of a 1986 law that allows warrantless interception of some digital data.

The unanimous decision (PDF), from a three-judge panel of the Sixth Circuit Court of Appeals, rejected prosecutors' arguments that there was no reasonable expectation that email is private when it's stored for more than 180 days. Such reasoning is antiquated today, when email conveys people's most guarded personal and business secrets and often lives on servers for years. As such, email should enjoy protection from unreasonable searches and seizures, the judges said.

“Given the fundamental similarities between email and traditional forms of communication, it would defy common sense to afford emails lesser Fourth Amendment protection,” the ruling stated. “Email is the technological scion of tangible mail, and it plays an indispensable part in the Information Age.”

The judges went on to declare part of the Stored Communications Act unconstitutional because it allowed the government to compel ISPs to turn over customer email without first obtaining a warrant based on probable cause.

Civil liberties advocates hailed the ruling.

“This is a very big deal,” Freedom to Tinker's Paul Ohm blogged. “It marks the first time a federal court of appeals has extended the Fourth Amendment to email with such care and detail. This is the opinion privacy activists and many legal scholars, myself included, have been waiting and calling for, for more than a decade. It may someday be seen as a watershed moment in the extension of our Constitutional rights to the internet.”

The ruling came in the criminal case of one Steven Warshak, a penis-enhancement marketer who was convicted of multiple fraud charges. In the course of the investigation, prosecutors accessed thousands of Warshak's emails without a warrant.

Tuesday's appeals ruling sent the case back to the lower court for a new sentence but upheld the conviction itself because police relied “in good faith” on their interpretation of the surveillance law. Warshak's 25-year-prison sentence was tossed on issues unrelated to the email seizure.

It's the latest in a string of victories for privacy advocates and comes on the heels of rulings that said the Fourth Amendment protects cellphone tower data and GPS tracking. ®