Some setting is wrong in your email setup. If you can't figure it out delete the email account, reboot the phone (hold HOME and SLEEP at the same time until an Apple logo appears; about 10-20 seconds), then add the account fresh.
I'v tried everything, from deleting my account, to setting yahoo up in other mail, I put in imap..mail.yahoo.ca incoming mail server smtp.mail.yahoo.ca as outgoing mail server but it keeps saying server not available
I am ready to pull my hair out. Everything worked perfect until last week
Please any help thank you