System Administrators, MVPs, please help!

Scenario:
I have a LAN that contains 15 computers. I'm using one of the computers
as a mail server (POP3). I'm running Windows 2003 Server SP2 with POP3 Service installed. I have two domains, let's say dept1.com and dept2.com, each consists of 7 accounts. Everything is fine. I recently decided to get a new computer and make it a POP3 server for the 2nd department. So, I have dept1.com on server1, and dept2.com on server2

My question: How am I gonna make people from different departments exchange messages? I can't send a message from an account in dept1.com to another account in dept2.com
Can we do it?

Awaiting your reply anxiously!
Thanks!