select distinct email, datetime
from Log
where email in (select email from tmp)
group by email;

| email                      | datetime            |
+----------------------------+---------------------+
| aaa@gmail.com              | 2014-02-06 14:08:28 |
| bbb@gmail.com              | 2014-05-22 18:53:39 |
| cc@gmail.com               | 2014-05-22 18:51:19 |
+----------------------------+---------------------+

select email, max(datetime )
  from Log
  where email in (select email from tmp)
  group by email;

select distinct email, max(datetime)
from Log
where email in (select email from tmp)
group by email;

select distinct email, datetime
from Log
where email in (select email from tmp)
group by email
ORDER BY datetime desc