Check the outgoing mail server setting. Make sure that your username and password are in there.
Settings>Mail, Contacts, Calendars>Your email account>Account>Outgoing mail server - tap the server name next to SMTP and check in the primary server and make sure your username and password are entered and correct - even if it says that the password is optional.
It might be helpful if you state who your email provider is so that someone with the same service can help as well.
You might also want to consider deleting the account from the iPad and add it back on again.
iOS: Unable to send or receive email
Can’t Send Emails on iPad – Troubleshooting Steps