code/authenticators/LDAPAuthenticator.php
protected static function fallback_authenticate($data, Form $form = null)
    {
        return call_user_func(
            [Config::inst()->get('LDAPAuthenticator', 'fallback_authenticator_class'), 'authenticate'],
            array_merge($data, ['Email' => $data['Login']]),
            $form
        );
    }
protected static function fallback_authenticate($data, Form $form = null)
    {
        return call_user_func(
            [Config::inst()->get('LDAPAuthenticator', 'fallback_authenticator_class'), 'authenticate'],
            array_merge($data, ['Email' => $data['Login']]),
            $form
        );
    }
Try to authenticate using the fallback authenticator.

@param array $data
@param null|Form $form
@return null|Member