Yahoo has had issues in the past with its email service that caused breaches in user accounts. A cross site scripting vulnerability in their servers allowed the possibility of session credentials to be hijacked. Which essentially allows access to a user's account without the password. So spam could be sent out to user's contact lists. Which is one reason I don't recommend using Yahoo Mail as primary email. As they were behind the times where security is concerned compared to other providers such as GMAIL.
