Configuring ISA Server Behind Pix 515e - Using RPC Filter

I want to set it up so users can access outlook from anywhere.  Not all users have Outlook 2003 so RPC over HTTP won't work.

I am going to have OWA as a fallback for users, but I want to allow my users in remote offices (which are not interconnected with us) to have access to their full MAPI Outlook client at all times.

I thought it out and here's what I've come up with:

Router - Pix - Internal Network - Exchange
              |
           DMZ - ISA Server (Hosting RPC Filter, Dual NICs - 1 to internal network, 1 to DMZ)

The problem I am having is that when I create the server publishing rule it is not recognizing my DMZ NIC address (192.168.2.2) as the external address for the server.

On the PIX the internal interface is 192.168.1.1/24 and the DMZ is 192.168.2.1/24.

Is it possible to set up my environment the way I laid it out or is their a better way to go about it?  I want to stay away from VPN clients and the remote offices are externally managed by an outside provider and I don't have access to the equipment.

Thanks.