SELECT
column1, column2, column3
 ,CASE
     WHEN column2 = '123-LK' THEN 'lk@email.com'
     WHEN column2 = '456-MM' THEN 'mm@email.com'
     WHEN column2 = '789-MT' THEN 'mt@email.com'
   ELSE 'default@email.com'
  END as column3
FROM dataext1