How to update one column with the value of another if one of their columns match
This will update all rows in the target table that match an email address in the same table that has been marked as 'do not email'

update a
   set a.note = 'do not email'
  from tbl as a
  join (select distinct email
          from tbl 
         where note = 'do not email'
       ) as b 
    on a.email = b.email
 where a.note &lt;&gt; 'do not email' 
    or a.note is null;