According to Reuters news agency, US based company Yahoo! last year secretly built a custom software program to search all of its customers' incoming emails for specific information provided by U.S. intelligence officials, according to people familiar with the matter.
The company complied with a classified U.S. government demand, scanning hundreds of millions of Yahoo Mail accounts at the behest of the National Security Agency or FBI, said three former employees and a fourth person apprised of the events.
Source:
http://www.reuters.com/article/us-yahoo-nsa-exclusive-idUSKCN1241YT