According to a new report by Reuters citing anonymous intelligence officials, in 2015, Yahoo covertly built a secret “custom software program to search all of its customers' incoming emails for specific information.”
Reuters noted that Yahoo “complied with a classified US government directive, scanning hundreds of millions of Yahoo Mail accounts at the behest of the National Security Agency or FBI, said two former employees and a third person apprised of the events.” It is not clear what data, if any, was handed over.
Presuming that the report is correct, it would represent essentially the digital equivalent of a general warrant—which is forbidden by the Fourth Amendment, as Electronic Frontier Foundation lawyer Andrew Crocker noted on Twitter.