Can't access Yahoo account on iphone- says 'IMAP server"apple.imap.mail.yahoo.com" isn't working'

My yahoo account has stopped working on my iphone 4s. I can access the account on a desktop but not the iphone. Comes up with 'The IMAP server "apple.imap.mail.yahoo.com" is not responding. Check your network connection and that you entered the correct information in the "incoming Mail Server' field.'

Any idea how to fix this? Hasn't happened before.