I think I have open nat because when I test my connection it doesn't warn me about anything when it's done. But there are still people on my friend's list I can't connect to. My other friends opened their ports so they could connect to them but I can't. I've setup a static ip and forwarded the correct ports for the xbox to it. I've also enabled the dmz for the xbox's ip so all connections should be allowed through the xbox's ip. I still cannot connect to everyone. I guess my nat settings are fine now but I would still like to be able to connect to certain people. Is there anything else I can do?

Try a different router.
I had to switch Routers back in the day because I was having problems with Halo 2.
Couldn't even join a party started by someone in the other room.