[mcnallym]

Where is the DNS for your public domain held. Is it on a server at your office or is it with your Domain Registrar on there server. From your original statement then I believe that your DNS Server is on yor Domain Registrars Server.

The DNS proxy on the firewall works by intercepting A Record DNS requests and responding with IP addresses configured in the DNS Proxy. It relies on having a proper DNS Server in your DNS that is responsible for resolving MX records or other non A Record requests. If it is not an A record request then the DNS Proxy ignores the request and allows the request to move onwards to the actual DNS Server. Looking at your original post then you have already figured out how to configure the DNS Proxy.

If the DNS is held at your Registrar on there DNS Servers then the requests will not be sent through the Firewall so the DNS Proxy cannot respond as it does not see the request, as these are all sent to the DNS Server at the Registrar.

One possible workaround would be to create a DNS Server inside the DMZ and statically NAT this through both ISP ranges. Configure the device to use ISP-1 DNS along with ISP-2 DNS as a backup, then if ISP-1 is down it gets no response and so sends to the ISP-2 DNS address and the request for the owa server would be intercepted. If you had a forwarder on the DNS server pointing at an Internet DNS Server then could look at web as normal.

This would be a work around however and require that whatever accesses the owa server uses your DMZ based DNS Server as there DNS Server, and I am not sure how practical to do for PC's as may not have permissions to do this. It will also require you to have a DNS Server in yoru DMZ. It would probably be just as easy to host your own DNS rather then have the Domain Registrars Servers do it.