I’m not one with real answers to this, but I don’t know that you can change it. I’ve gotten spoofed spam from well-known domain names before, and when I contacted the owners of those domains, they tell me there’s nothing they can do. Hopefully, someone here will say I’m wrong and have a real answer (I’m interested, too).
I don’t know if this would affect it, but do you have a catch-all address set up for your domain? If so, you shouldn’t. When setting up addresses, it tells you that a catch-all address may cause you to receive all kinds of spam.