to route specific dns query to specific dns server

[Riccardo Castellani]

I'm using Bind9 for my name server (SERVER EXT) and to give name resolution 
for who access from Internet to my domain (e.g. to access to my Web site or 
to write to my email addresses).
My domain is example.com:

www.Example.com
test.host at example.com

This dns server maps only my pubblic addresses.
This server has 2 nics: internal + external ip address.
Some internal servers, as proxy or mail servers, send dns requests to this 
dns server to solve names.
I have also internal MS domain (dns server is SERVER INT)  which is 
different from the other, it's created by Domain Controllers + AD 
(activedirectory.com) and it's used to map machines into internal network.

Now I my email server or proxy server (which are in internal network) need 
to synchronize time so they have to use my internal NTP server; these Linux 
machines use 'SERVER EXT' in /etc/resolv.conf, so how I can indicate to send 
request for specific internal name (ntp.activedirectory.com) to dns server 
INT ?
I could insert it inot /etc/hosts but it's not dnssssss service !!!