Was Yahoo's mass wiretap legal?

Yesterday, a very aggressive and very secret surveillance operation was abruptly made public. According to an exclusive Reuters report, Yahoo complied with a government order last year asking the company to scan all incoming emails for a specific string of characters — effectively surveilling hundreds of millions of users in search of a single phrase or snippet of code. In a statement this morning, Yahoo called the report "misleading" and emphasized that the mail-scanning system does not c