SELECT *, T.id FROM policy RIGHT JOIN
(SELECT
    NULL AS id, 0 AS priority, NULL AS policy_id, email, NULL AS fullname
    FROM domains
UNION ALL SELECT
    *
FROM
    users) AS T ON policy.id = T.policy_id
WHERE email IN('postmaster@webdevguru.co.uk', '@webdevguru.co.uk', '@.')
ORDER BY T.priority DESC