To ensure that your Exchange server will be able to contact external mail servers and vice versa, you must ensure that Domain Name System (DNS) is on your network and can resolve Internet domain names. Use this checklist to verify that resolution is taking place correctly:
Assign a public IP address provided by your Internet service provider (ISP), public vendor, or host site to your ISA server's external network adapter.
Define your public Mail Exchange (MX) and Host (A) records and map them to the ISA server's external IP address.
Verify that your external domain is registered publicly—that is, you can look it up at a site like www.allwhois.com, or perform an Nslookup-type=mx domain command to see the IP addresses registered for your domain.
For detailed steps for configuring a DNS environment for your Exchange and ISA Server environment, see Chapter 4 in the "ISA Server 2004/Exchange Server Deployment Kit," at http://www.microsoft.com/isaserver/techinfo/guidance/2004/configuration.mspx.