Query for frequency of e-mail addresses in a field
I think you want a "count of counts", which can be done with 2 GROUP BY, once to find how many times an emailaddress has been used: 

    SELECT emailaddress, COUNT(*) AS frequency
    FROM tablename
    GROUP BY emailaddress ;


and then enclosing the above in a derived table and doing another GROUP BY to find how many emailaddresses were used once, twice, thrice, etc...:

SELECT frequency, COUNT(*) AS emailadresses
FROM
  ( SELECT COUNT(*) AS frequency           -- we don't need email any more
    FROM tablename
    GROUP BY emailaddress
  ) AS x 
GROUP BY frequency ;




Minor mysql quirk. If you want:

GROUP BY frequency 
ORDER BY frequency DESC ;


it can be abbreviated (and it will be a bit more efficient) in MySQL as:

GROUP BY frequency DESC ;