Prevent User nobody from Sending Mail

To prevent spammers from sending mail through vulnerable scripts or uploading and running their own scripts to send spam from your server, you may want to prevent the user nobody from sending mail from the server. The users on the server will still be able to have scripts that send to their local addresses. If they want the script to send to an outside address, they can create a forwarder and then have their script send mail to the forwarder.

This security feature is enabled by going to Tweak Settings in the Server Configuration section of the WHM. In the Mail section, check "Prevent the user "nobody" from sending out mail to remote addresses". Finally, scroll to the bottom and click Save.