DNS can't resolve a domain name

[moller]

Hi
I got a problem, I have 4 windows NT 4.0 servers use ISP's DNS one of these servers is Exchange 5.5

One day I couldn't send two e-mail address(the undelivery message is host unknow) , then I use nslookup to check it. The response from my NT servers was "Domain name non-existent"
then I use other DNS the result was "no response from that server" this result happended in all NT server.

then i asked my friend (out of my office) used nslookup "he could resolve the IP address" with same DNS server.

then I edit host file in exchange server and I could send e-mail

Any mis-configured in my NT server ??

I install a PIX firewall is that any problem with my firewall?