problems with firewall, DMZ

Hi,
I have got a pair of 2 public IPS to test from my ISP. I have assigned both for my gateway machine and one of them is Natted to a test mail server at DMZ. I can access the mail server from outside world as, telnet <external ip> 25
it works fine

My internal users can access it as,
telnet 10.10.10.2 25
it works fine,

But I cannot do
telnet <external ip> 25
from inside.
Same for POP3 and HTTP.
I cannot do it even from the gateway(firewall) machine itself.

What must be the problem?

With warm regards,
-Payal