ドメインごとでGROUP BYするには substring_index を使います。
SELECT substring_index(email, '@', -1) domain, COUNT(*) email_count FROM turbine_user GROUP BY substring_index(email, '@', -1);
参考
https://www.firstlogic.co.jp/blog/lab/naruhiko/5237
http://stackoverflow.com/questions/2440429/mysql-query-to-count-unique-domains-from-email-address-field