Microsoft, one of the largest e-mail hosts in the world, is challenging a US federal judge-ordered search warrant that would compel the company to hand over customer e-mail data stored on its overseas servers.

In a legal brief (PDF) filed in US federal district court in New York last Friday, Microsoft objected to the judge’s order to turn over to government snoops e-mail data contained on the company’s servers located in Dublin, Ireland. The information sought by the court relates to an ongoing drug trafficking investigation; the precise e-mail account remains redacted in the court record.

Microsoft’s attorneys argued that information stored on the company’s servers in Ireland should be subject to Irish law, reasoning that absent Irish court approval, such foreign servers should be inaccessible to US courts.

“Congress has not authorized the issuance of warrants that reach outside US territory,” Microsoft’s attorneys wrote. “The government cannot seek and a court cannot issue a warrant allowing federal agents to break down the doors of Microsoft’s Dublin facility.”

The attorneys argued that the warrant the company received was overbroad and thus in violation of the Fourth Amendment of the US Constitution's "particularly requirement," whereby information requested in a warrant must be specifically identified.

While Microsoft did not produce the content of the e-mail to the government, it did hand over "non-content information and address book data that was located in the United States."

Microsoft told the US government in its 38-page brief that the US should "seek the relevant user information by following the process established by the US-Ireland [Mutual Legal Assistance Treaty]."

In a declaration (PDF) to the court filed last Friday alongside the brief, Michael McDowell, a former attorney general of Ireland, said that such evidence handed over pursuant to the treaty was "efficient and well-functioning."