Do you have yahoo email set up in your email app, whatever it is? (If this was someone else's phone, and they had a yahoo account, and put it into the email app, they would have changed their password when they realized what they'd done, so imap.mail.yahoo.com won't work. And you probably have your email app set to check email every 30 minutes (which you shouldn't - "never" or "0" let's it receive email as it comes in).