Dirt on rear end (Allgemein)

Hello.

On the rear end of my Alpinweiss 525i there's always kind of a brown/yellow "versmutzung". If I'm correct this is caused from "dirty" exhaust gasses, cars with cat don't seem to have this trouble. It's pretty stubborn stuff and can only be removed by polishing, but it always returns after some time. It's looks dirty, and all that polishing all the time isn't good either....

What can be done about this? Will a chrome exhaust tip help maybe?

Thanks.