EX2K on Lan and SMTP on DMZ.

I have my EX2K collocation installed on Lan (W2k adv) and a standalone Win2K advanced server running smtp on dmz of the Symantec Firewall. I want email messages to go from senders to the dmz server first then it forwards messages to EX2k on lan. People who work for the company will receive emails from the lan email server (outbound email will come out directly from FW). As of right now, I have a hard time with smpt setup on dmz email server and outbound email to communicate with the internet.
Please, help in details asap and you can get whatever points you want.
Thanks for your time.