Between September 2014 and March 2016, Microsoft says, it received 5,624 demands for customer data from the Office 365 cloud suite and other services from law enforcement. Of those, 2,576 - almost half - were accompanied by "secrecy orders," meaning that Microsoft wasn't allowed to tell the customer or anybody else that law enforcement had access to their data.
In fact, Microsoft says, 1,752 of those secrecy orders came with no time limit, meaning that they will never be allowed to inform the customer.
In the court filing, Microsoft writes:
Microsoft brings this case because its customers have a right to know when the government obtains a warrant to read their emails, and because Microsoft has a right to tell them. Yet the Electronic Communications Privacy Act ("ECPA") allows courts to order Microsoft to keep its customers in the dark when the government seeks their email content or other private information, based solely on a "reason to believe" that disclosure might hinder an investigation.
These matters of privacy and informing the customer have never been more timely: Apple's high-profile clash with the FBI is still fresh on everybody's mind. But if Microsoft wants to challenge the constitutionality of this law, it likely has a long, hard fight ahead of it.