Go into "Settings">"Mail,contacts..."> tap on your "Yahoo account">tap on "Account" (the one with your email address after it.). Now go down to "Incoming Mail Server" it should read imap.mail.yahoo.com, if not change it, then goto "Outgoing Mail Server", "SMPT" should read smpt.mail.yahoo.com , if not change it, that should fix it. Setup has been putting the wrong things in these fields, and it drove me crazy at first. If that doesn't do it, find someone with a yahoo account, that works, and compare those entries. If it doesn't, good luck.
