select email
from Email_Table t1
where not exists (select email
                  from Contacted_table t2
                  where t1.email = t2.email)

select email
from Email_Table t1
where email not in (select email
                    from Contacted_table)

SELECT email FROM email_table e
LEFT JOIN contacted_table c ON e.email = c.email
WHERE c.email IS NULL

SELECT users.email, contacted.email
FROM users LEFT JOIN contacted ON users.email = contacted.email

users.email | contacted.email
-----------------------------
aa@aa.com   | aa@aa.com
bb@bb.com   | bb@bb.com
cc@cc.com   | cc@cc.com
dd@dd.com   | NULL
ee@ee.com   | NULL

SELECT users.email
FROM users LEFT JOIN contacted ON users.email = contacted.email
WHERE contacted.email IS NULL