iOS: Unable to send or receive email
Can’t Send Emails on iPad – Troubleshooting Steps
Setting up and troubleshooting Mail
Server does not allow relaying email error, fix
Why Does My iPad Say "Cannot Connect to Server"?
iOS: 'Mailbox Locked', account is in use on another device, or prompt to re-enter POP3 password
Configuration problems with IMAP e-mail on iOS with a non-standard SSL port.
Try this first - Reset the iPad by holding down on the Sleep and Home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider - let go of the buttons. (This is equivalent to rebooting your computer.)
Or this - Delete the account in Mail and then set it up again. Settings->Mail, Contacts, Calendars -> Accounts Tap on the Account, then on the red button that says Remove Account.