TS2755: iOS: Troubleshooting MessagesLearn about iOS: Troubleshooting Messages
it is possibly a password issue from your smtp settings
under settings-mail,contacts,calendars- select the account you are having problems with -then select outgoing mail server-primary server- fill in any missing password info
iOS: Unable to send or receive email
Can’t Send Emails on iPad – Troubleshooting Steps
Try this: Delete the account in Mail and then set it up again.