when you are absolutely sure that the password is correct (Capitals, non-capitals), then the problem may be in the Keychain.
In Applications/Utilities/KeychainAccess click "Keychain First Aid" in the main menu. Type your administrator password and click "repair". After the repair Quit the Keychain app.
Quit the mail app. Restart the Mail app, it will ask you again for your password, enter it correctly.
Normally this will not be asked again.
If you still have the issue, a "harsher" method is deleting your account completely and after restart of Mail, adding the account again with the correct settings.
iOS: Unable to send or receive email
Can’t Send Emails on iPad – Troubleshooting Steps
Setting up and troubleshooting Mail
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.
Or this - Delete the account in Mail and then set it up again.