If you are running PHP script as an Apache user on cPanel server, it is difficult to find the account if someone sends large amount of emails via PHP script. It is due to the fact that all emails will be sent as a nobody user. cPanel provides settings to block emails which are sent via nobody user. To enable this option, follow the below steps:
– Login to your WHM.
– Go to Server Configuration -> Tweak Settings.
– Select option “Prevent the user “nobody” from sending out mail to remote addresses (PHP and CGI scripts generally run as nobody if you are not using PHPSuexec and Suexec respectively.)” and save it.
Please note if PHP processes are running as a nobody user and you enable above option, your clients will not be able to send emails outside their domains using PHP mail() function.