Cisco Firewall :: 5510 Exchange Active Sync Stops Working

I know that I've run into this before but I can't remember the fix.  I have a 5510.  The 3 interfaces involved are INSIDE, OUTSIDE, and GUEST. Corporate users are allowed to put their iPhones on the Guest network, but the problem is that their Exchange ActiveSync stops working.  It is tied to the external DNS name of the OWA server (we'll say webmail.abc.com).  So the users are funneled out one public IP on the OUTSIDE interface and are trying to communicate with the outside of the OWA server, which is NATed to another public IP on the same outside interface.  What do I need to do on the ASA to allow users on the guest network (behind the GUEST interface) to access the mail server using its public IP (behind the INSIDE interface)