id | email        | domain
---|--------------|--------
1  | foo@bar.de   | bar.de
2  | baz@bar.de   | bar.de
3  | admin@bar.de | bar.de
4  | info@bar.de  | bar.de
5  | bar@foo.com  | foo.com
6  | baz@foo.com  | foo.com

select t.*
from thetable t
where (select count(*)
       from thetable t2
       where t2.domain = t.domain and t2.id <= t.id
      ) <= 2;