We've found that iOS can be a little buggy when it comes to accepting changes to your Mail settings. Try closing the app and power cycling your iPhone, then reopen Mail. If this does not work, try deleting your email account on your iPhone, power cycle the iPhone, then set the email account up from scratch.
Important Please ensure that you back up any email data that you're storing locally on your phone before you delete any accounts. If you're using IMAP, your mail will be stored on the server and not your phone.