Ok, go into settings, mail, contacts & calendars, scroll down through mail until you see default account, note what it is, scroll back up to the top where it says accounts and select the account you just noted. Scroll that down to advanced and select mail, scroll that down to outgoing mail server and see what that says, tap on the server and see if you get any others, if there are no others or they are switched off you will need to either switch them on or add a new one, if you have enabled your Appleid you should already have an iCloud SMTP server defined so you should be able to use that.