From your comments, I understand that you would like to configure the PIX to allow hosts in DMZ to relay mail to inside Exchange Server.
Use static (inside, dmz) command which will let any user in the inside network access the dmz and another one to allow any device in the dmz to access devices on the inside. Also configure an access list on dmz interface to allow smtp.
You would also need to configure an access list to permit ip so that the devices on the dmz network can go out to the internet and also in to the inside network using any ip port.
