The following MySQL query is useful for getting a count of all the domain names used by email addresses in your database. It isolates the domain name (after the @ sign) in the email field.
SELECT substring_index(email, '@', -1), COUNT(*) FROM login GROUP BY substring_index(email, '@', -1);
It can also be used to track down spammers or disabled accounts:
SELECT substring_index(email, '@', -1), COUNT(*) FROM login WHERE status = 'disabled' GROUP BY substring_index(email, '@', -1);
Link to MySQL String functions reference.
Link to MySQL substring_index() function.
When I find a useful little snip like this I like to share it (and document it in my blog so I can remember it)!
Post inspired by: http://stackoverflow.com/questions/2440429/mysql-query-to-count-unique-domains-from-email-address-field