Instead of opening a new thread, I'm going to bump this one.
This has been a long-standing cPanel problem. Forwarders can cause massive problems because they are not spam-filtered. If you set up a forwarder to a gMail/Hotmail/AOL account, it won't be long till email from the server has trouble reaching the mailbox, because they see the server as the source of all the forwarded spam.
It would be great to have a solution to this already...
Yes, the feature request doesn't seem to have a lot of traction, but there are a lot of threads in the forum about this same issue.
The problem is that, as hosting providers, we face this problem a lot. It can be summed up with this customer's question: "How come I can have a free forwarding service from my domain registrar, and forward email to anywhere I choose, and I can't do that with your paid cPanel hosting service?"