Exchange behind Cisco PIX firewall

Good morning all,
We have a simple network running a Cisco PIX 515ur firewall with our Exchange server residing in the DMZ and the main (token ring) network behind a Proxy server. The only router is outside the PIX. We had been experiencing extremely long receipt delays and duplicate messages. Research pointed to the MailGuard feature on the PIX and I have disabled it as of this morning.
My concern now is the protection of our Exchange server. I am checking the MS KB but would like to hear what some of you have experienced/implemented.

Happy Holidays and Thank you. Carolyn