If the SMTP Restrictions setting in WHM is turned off, it is possible for users on the server bypass Exim and all outgoing spam checks. When this happens spam messages can be sent from your server and there would be no logs or method to identify the source of the spam. It is highly recommended to enable this setting.
NOTE: When this setting is enabled, all websites and scripts must be configured to use your cPanel server as the SMTP host, and they must use a proper email address and password to authenticate. Details about how set this up for your scripts can be found here:
1. Login to WHM as the root user
2. Navigate to: Home »Security Center »SMTP Restrictions
3. Check this page to see if it says that the option is already enabled.
4. If you see "The SMTP restriction is disabled.", click the blue "Enable" button.
Now all messages that exit the server (except those sent by root) must first go through Exim where they will be checked and recorded.