Email keep failing to send from iPhone 5s. help!!!!!!

My outgoing mail keeps on not sending. In my settings its going through port 25. Password is correct and on. Mail comes back saying rejected by the server. I use blueyonder email. Sometimes it says password or username are incorrect but theyre not. Ive deleted my email account on the iPhone and then re added it but still no joy. Ive tried changing the port but nothing else works. Does anyone else have these problems? Its a right pain in the bottom! Please can anyone advise me?

Many thanks in advance xxx