Yahoo secretly built a tool to scan all email in realtime for US spies

In 2015, Yahoo CEO Marissa Meyer ordered the company's engineers to build a tool that scanned Yahoo Mail messages in realtime for "characters" of interest to a US security agency, either the FBI or the NSA.

As far as we know, the move was unprecedented in US tech history, and it triggered the departure of then-CSO Alex Stamos (previously), now CSO for Facebook.