PIX 515E Exchange on Inside interface, SmarterMail on DMZ

In my config I am running a PIX 515e Firewall with a domain and exchange server running on the inside interface. I have web servers and smartermail server running in the DMZ. Everything works except sending email from SmarterMail email account to Exchange email account on inside interface. It to do with the public ip of the exchange server not resolving to the SmarterMail server. Can anyone help with this. I know I am missing one simple rule in pix and for the life of me cannot figure it out. Thank you.