If for some reason or other you need to find out who is receiving system emails: here's a neat trick on how to find these. Go to your phpmyadmin, find the _users table and Search in the #_users table in the database where sendEmail=1

So basically you'll need to run the following SQL query using phpmyadmin

SELECT *
FROM `xxxx_users`
WHERE sendemail=1

The results of the query will be those users who are enabled to receive joomla 3 system emails!