SELECT substr(email,locate('@',email)+1,255) AS host,count(1) AS count
FROM my_emails
WHERE email like '%@%'
GROUP BY substr(email,locate('@',email)+1,255)
ORDER BY count(1) DESC
LIMIT 10

db.my_emails.aggregate(
    {
    $group : {_id : "$host", total : { $sum : 1 }}
    }
  );