DNS Server

I have a situation where is my company, our exchange server are connected directly to the internet via lease line using 256 bandwith that connected to Sisco router(bridgehead)for x400 connector.The problem is when we put the internal DNS server in this exchange server DNS config. we cant resolve most of the internet pages but when we use external DNS, we are able to see the pages. IS anything wrong with my DNS server? I always get the error message that i have already stated in my last questionabout the DNS.