Five years ago, US law enforcement served Microsoft a search warrant for emails as part of a US drug trafficking investigation. In response, Microsoft handed over data stored on American servers, like the person’s address book. But it didn’t give the government the actual content of the individual’s emails, because they were stored at a Microsoft data center in Dublin, Ireland, where the subject said he lived when he signed up for his Outlook account. In a case that begins Tuesday, the Supreme Court will decide whether those borders matter when it comes to data.

U.S. v. Microsoft, which hinges on a law passed decades before the modern internet came into existence, could have broad consequences for how digital communications are accessed by law enforcement, and for the nearly $250 billion cloud-computing industry.

"The case is hugely important, it has implications for the future of the internet," says Jennifer Daskal, a former Justice Department official who now teaches at American University Washington College of Law. The case is primarily about "whether we update our laws regarding access to information for the internet age,” she says.

The Emails in Ireland

As the case has worked its way through appellate courts, Microsoft has taken the position that US law enforcement needs to go through Irish authorities if they want to obtain the emails. The United States has a Mutual Legal Assistance Treaty with Ireland, as it does with over 60 other countries and the European Union. Microsoft holds that US law enforcement could simply use the MLAT to ask Irish authorities for help.

The Justice Department argues that the warrant issued in the US should suffice, without needing to deal with Ireland to obtain the emails. It says the warrant is valid not because it has international reach, but because the actions required for Microsoft to obtain the data could take place within the United States. In other words, the government is saying that copying or moving the subject’s emails stored in Ireland isn’t search and seizure—only directly handing the emails to the US government is.

'Countries around the world would be insisting that their legal process compels Microsoft and other providers to disclose data that they hold in the United States.' Gregory Nojeim, Center for Democracy & Technology

Organizations like the ACLU, Brennan Center for Justice, and the Electronic Frontier Foundation all filed an amicus brief to the Supreme Court arguing that the government’s logic relies on an erroneous interpretation of the Fourth Amendment. “A company acting as a government agent is conducting a Fourth Amendment ‘search and seizure’ when accessing, copying, or moving a user’s data, regardless of when, where, or even whether investigators later search it,” writes Jennifer Stisa Granick, surveillance and cybersecurity counsel at the ACLU’s Speech, Privacy and Technology project.

Microsoft argues the case has to do with digital privacy. “We believe that people’s privacy rights should be protected by the laws of their own countries and we believe that information stored in the cloud should have the same protections as paper stored in your desk,” Brad Smith, Microsoft's chief legal officer, wrote in a blog post published in October, when the Supreme Court first agreed to hear the case. "The U.S. Government argues that it can reach across borders based on a law enacted in 1986, before anyone conceived of cloud computing. We don’t believe there is any indication that Congress intended such a result," Smith wrote in another post published Tuesday.

The company and privacy advocates also argue that if Microsoft Ireland results in an adverse ruling, the US government wouldn’t be able to reject demands from other countries for communications stored on US soil. "Countries around the world would be insisting that their legal process compels Microsoft and other providers to disclose data that they hold in the United States, which would result in chaos," says Gregory Nojeim, senior counsel and the director of the Freedom, Security, and Technology Project at the Center for Democracy & Technology.

The Trump administration, which inherited the case from Obama, contends that if Microsoft wins, US law enforcement will lose the ability to easily obtain evidence related to serious crimes, like child pornography and terrorism. They worry that companies could easily shift their data beyond the reach of US authorities by simply moving it out of the country. Even using MLAT agreements can be cumbersome, especially if multiple countries' laws come into play. Google, for example, sometimes separates files into multiple pieces, which are stored in different places and constantly shuffled around. A Microsoft win might make it difficult to say, obtain both the emails and the photos in a child porn case, argues the government, if they're stored in different countries.