A Mail configuration problem

["Soumeyndu S. Mohapatra" <ssmohapatra@xxxxxxxxxxxxxx>]

Dear friends;
I have problem in configuring SendMail in following Scenario and
Requirment.I am a novice in linux configuration and I don't know whether
it is practically feasible or not.

SCENARIO
--------
1. We have a private network which is separeted from internet by firewall.

2. we have registered a domain name xyz.com and all our employee have mail
   id like abc@xxxxxxxxxxxx usally access their mail by using pine
   software which is configured to IMAP access to mail server of xyz.com
   domain.Disadvantage is that pine takes a lot of time for
   authentication and link of our network to Remote mail server is very
   slow.The remote mail server is separated by a very slow WAN link.
   We are able to access the remote mail server only by IP address.
   The inbox of pine is configured like {202.183.1.7}inbox

3. I have configured a linux m/c with a primary DNS(unregistered) for
   pqr.com.I have configured linux so that users of linux can exchange
   mail using user@xxxxxxx locally.This linux m/c is connected to
   network(but not to internet).It doesn't have capability to resolv
   either internet domain name nor xyz.com domain name.
Requirment
----------
1. Local Linux m/c users should be able to send mail to outside world as
   well as pqr.com domain.Local mails should be delivered locally without
   going out.Mails to pqr.com domain should be delivered to pqs.com.
   Mails to Internet domains should be delivered.If any local user sends mail 
   to outside world then mail should go with xyz.com domain name.Local users 
   should able to down load mails from remote mail server.

I have tried my best to explain the problems.If any further clarification
required please write me. wating for solution
ssmohapatra@xxxxxxxxxxxxxx