Just yesterday I started to get this error message Error Code: 500 Internal Server Error. Forefront TMG cannot handle your request because the DNS quota was exceeded. Contact your Forefront TMG administrator. (12218) while browsing the internet. I could not find any information on this message at Microsoft or anywhere else on Google. Well. I think I know what it was. It has to do with 2008r2's DNS not responding to all requests after a while. I had changed all sorts of things trying to get it to work (including rebooting). Flood mitigation, disable dns filter, disable http proxy. Pretty much took TMG down to nothing and it didn't fix it. I rebooted the Domain Controller (which hosts the DNS that the TMG box talks to) as well as the TMG server again. Then poof, it worked. So, my solution to this problem, reboot everything, DNS server, and TMG server (not just services). Then I put all the settings on TMG back to the way they were before and the problem has still not showed up. At least now there will be something on the internet about this error that just might help someone out in the future.