Yahoo
This is a warning to everyone. If you want to be secure and use a free email service from Google, Yahoo, Microsoft or others then make sure to encrypt your email from end to end. You can usually do this by searching for it in your security or privavcy settings.
Here is the Denver News 7 piece:
http://www.thedenverchannel.com/thenow/report-of-millons-of-emails-secretly-scanned-prompts-cyber-privacy-questions