Microsoft has asked a US court to block the country's government from forcing it to hand over customer emails stored on an Irish server.
Authorities have issued a search warrant seeking the emails of an unidentified individual in Dublin, as part of a drug investigation, but the software company has challenged the warrant arguing that it could set a precedent that would create a "global free-for-all" hugely damaging to personal privacy.
Upholding the warrant would open the door to other countries using their law enforcement powers to seize the emails of Americans held in the US, Joshua Rosenkranz, a lawyer for Microsoft, warned the 2nd US Circuit Court of Appeals in New York.
"We would go crazy if China did this to us," he said.
The case has garnered widespread attention from the technology industry and privacy advocates and nearly 100 organizations and individuals have filed briefs in support of Microsoft's position, including tech giants like Apple, Verizon Communications and Cisco Systems.
The question under consideration by the three-judge panel is whether the warrant is an "extraterritorial" application of the law or whether the fact that US-based Microsoft employees can retrieve the data means the statute is being used only within the US.
The statute at issue is the Stored Communications Act, which was enacted in 1986, decades before providers like Microsoft began building servers abroad to improve speed.
Justin Anderson, a lawyer for the government, said US law enforcement can obtain electronic information held by American companies with a valid warrant, regardless of where the data happens to be stored.
"It's not a question of ownership," he said, likening it to seizing account records from a bank. "It's about custody and control."
Rosenkranz said the court should leave it to Congress to close what he said was a gap in the law, but Circuit Judge Gerald Lynch said that, while legislative action would be best, Congress does not always act quickly. "We'll all be holding our breath," he added.