Microsoft initially challenged the order, saying that local laws must apply in respect of each jurisdiction.
Microsoft's general counsel Brad Smith said in remarks following the ruling:
“The only issue that was certain this morning was that the District Court’s decision would not represent the final step in this process. We will appeal promptly and continue to advocate that people’s email deserves strong privacy protection in the U.S. and around the world.”
Smith argued in the Wall Street Journal on Tuesday that the US government "can't force American tech companies to turn over customer emails stored exclusively in company data centers in other countries."
"Microsoft believes you own emails stored in the cloud, and that they have the same privacy protection as paper letters sent by mail. This means, in our view, that the U.S. government can obtain emails only subject to the full legal protections of the Constitution's Fourth Amendment," he said.
But because the case rests on data stored by Microsoft in its Dublin, Ireland-based datacenter, that data should also fall under the purview of Irish and European data protection laws, of which Ireland is a member state.
Verizon came to a similar conclusion in a blog post by its general counsel Randall Milch in January. "The U.S. government cannot compel us to produce our customers' data stored in datacenters outside the U.S., and, if it attempts to do so, we would challenge that attempt in court," he said in a blog post.
New proposals set to come into force following extensive scrutiny and voting later this, or next year, will reform Europe's data protection laws. These proposals seek to prevent a European subsidiary of a parent company, such as in the US, from handing over data to a third-country for law enforcement or intelligence purposes.
European authorities have repeatedly said, regardless of where a EU-based company's parent is headquartered, that subsidiary must abide by European law.
Falling foul of that could result in a breach of European law, and therefore international law, EU Justice Commissioner Viviane Reding previously told ZDNet.