DNS Name Resolution Issues with TMG
You might already know this, but just in case you didn't, the TMG firewall will perform name resolution on behalf of Firewall and Web proxy clients. It then keeps a cache of those name resolutions which can speed things up quite a bit. But sometimes there can be problems. This blog post describes a problem that one TMG firewall administrator had.
http://snowvm.com/2013/05/23/buggy-dns-resolution-using-microsoft-forefront-tmg-2010/
HTH,
Deb