I think most of the public email hosts have some sort of monitoring. Mostly for spam and dangerous content. I don't think anyone actually reads the emails. That would seem to be illegal invasion of privacy. I think they all use some sort of automation programming that searches content for dangerous active links or attachments, known isp's of spammers where volumes of emails are generated, hackers and such.
No, I don't think you're in trouble. The gov't can subpoena emails and other internet searches that are done. Search engines generally comply. You can read about yahoo policies by doing a search. Many sites discuss it.
Here's a link
http://www.hrichina.org/public/highlight/yahoo.html
You can look at the US items.