Fai quest'ultimo tentativo, altrimenti posta un dump significativo.

codice:
set @mail := '', @num := 1;
select * from (
select *,
   @num := if(@mail = mail, @num + 1, 1) as riga_numero,
   @mail := mail as m
from tabella
order by mail asc, punteggio desc) as tab where riga_numero = 1;