In order to handle address resolution for both internal and external hosts, you'll need to configure a split DNS configuration. A split DNS is a configuration where queries from inside the firewall are directed to a DNS server located inside the firewall, which is authoritative for those internal addresses. This DNS server will return the "inside" addresses of other servers on the local network, and will forward any requests for "outside" addresses to the main DNS server. You can configure this by setting up the A and MX records for the internal zone within a DNS server running on the ZCS server. This will cause the local hostnames to resolve to the correct internal addresses.