NAT server and Microsoft exchange.

Previously, they used PIX firewall (cisco) as a gateway and now I have setup a NAT server as a gateway on Windows 2003 server. The NAT is working, well and the client are able to use Microsoft exchange server internally but cannot sent or receive massege outside the LAN. Please, I am confused on what step to take next and I am new to Microsoft exchange server, I would appriciate any advice or idea on this issue. thanks to all