When are your private emails really out of the U.S. government's reach?

Microsoft thinks it knows the answer and is fighting in court to prove its point.

At a press gathering in New York on Monday, Microsoft (MSFT) laid out its case for why it is refusing to comply with a federal subpoena and search warrant for some customers' emails. The emails live in a data center in Ireland. And Microsoft argues they should be off-limits to the U.S. government until Ireland gives the OK.

Apple (AAPL) and 18 other tech rivals, as well as 16 media organizations, including CNN, have filed or said they will file court briefs in support of Microsoft's position.

Microsoft's argument basically boils down to this: Data stored overseas is not subject to U.S. law enforcement. If Uncle Sam wants information that lives in a foreign server farm, it has to go through a legal channel known as a mutual legal assistance treaty.

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For example, just because Hilton is a company based in the United States, the U.S. government couldn't search a Hilton hotel room in England. Items in that room would be covered by U.K. laws, noted Ed Lazowska, computer science professor at the University of Washington, who spoke during the briefing. (Lazowska's department is sponsored by Bill Gates, Microsoft's founder).

But in August a federal trial judge rejected Microsoft's argument. The judge's reasoning: Since the emails can be instantly transferred to the United States with a click of a button, the search would actually occur here. Microsoft has appealed the decision.

Details about the criminal case have been sealed, but Microsoft hinted that the customers in question are not American. The company noted that it typically houses data in server farms that are close to the customers themselves in order to minimize upload and download lag times.

"If this were a matter of Americans trying to evade U.S. law, that would be a different question," Brad Smith, Microsoft's chief counsel, said Monday. "Should we tell Europeans that they are now subject to U.S. law? That doesn't seem like the right precedent."

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Smith acknowledged that Microsoft's bottom line is at stake. The cloud computing business is booming. People and corporations around the world are increasingly storing their data in server farms owned by Microsoft, Google (GOOGL), Apple and others. Customers must believe that their emails are secure.

But Smith also said Microsoft believes it is being principled about the matter. If Ireland were to give the OK, Microsoft would turn over the data without question, he said.

So why haven't U.S. prosecutors just asked Ireland for the emails. Too much red tape, the U.S. government argued. In addition, Ireland is a U.S. ally -- what happens when the servers are in countries with unfriendly governments?