I've read the boards reguarding this topic, and the use of :fail: in the mail filtering part of Cpanel. However, I can't seem to get any mail to bounce back whenever I test it. The test script at the bottom of the page simply says that it is being sent to /dev/null.
Is the :fail: function supposed to bounce an email back to a sender as if the email address didn't exist in the first place? Or is this a job for an external script?