If and when you are using Yahoo webmail and have been prompted to what they call "upgrade" their services I recommend reading the associated fine print:
Yahoo Terms Of Use said:By clicking the button above, I certify that I have read and agreed to the Communication Terms of Service, and Yahoo! Privacy Policy. To deliver product features, relevant advertising and abuse protection, Yahoo!'s automated systems scan and analyze all email, IM and other communication content.
Yahoo said:This technology looks for patterns, keywords, and files in Mail, Messenger, and other communications content.