DNS problem on Linux

Hi,
We have one mail primary DNS. Behind our institutional firewall we have
one more DNS. This has a number of zones defined for internal IP
resolution. We expect it to work as when somebody tries to resolve
X.mnit.ac.in the request goes to mnit.ac.in and then it will forward
the request (on the basis of CNAME) to internal DNS for resolution.
When we dig the X.mnit.ac.in at the internal DNS (or host X.mnit.ac.in)
it returns the correct internal IP but when somebody tries to access
from outside our campus through name only X.mnit.ac.in it does not
resolve. How can I resolve this problem?
Many thanks in advance
karmath