Okay I found it although my search feature isn't working in firefox. I scrolled down again and under a button that reads "Add Additional Configuration Setting" there it was. Not sure how I missed it the first time but oh well, I added the code from the thread to the field under Section: BEGINACL. Hopefully this tweak will do what I want it to. We will see if I keep getting undeliverable mail returned messages from the servers who receive the email from the culprits who are spoofing my email address.
Thanks to cPanelMicheal and Infopro for your help and patience.