Validate email addresses in Mysql

You can use a pure SELECT to validate Email Addresses:

SELECT * FROM `users` WHERE `email` NOT REGEXP '^[^@]+@[^@]+\.[^@]{2,}$';

And now for your question of tracking multiple tables, you can use comma seperated table names right?

SELECT * FROM `users`, `customers`, `clients`
         WHERE `email` NOT REGEXP "^[a-zA-Z0-9][a-zA-Z0-9.!#$%&'*+-/=?^_`{|}~]*?[a-zA-Z0-9._-]?@[a-zA-Z0-9][a-zA-Z0-9._-]*?[a-zA-Z0-9]?\\.[a-zA-Z]{2,63}$";