"Host Unknown" message when sending outbound Internet mail

All outbound Internet mail may be returned with a message that is similar to the following:

This issue can occur if the Microsoft Windows 2000-based Domain Name System (DNS) server that Exchange 2000 or Exchange Server 2003 is using contains a forward lookup zone for the root domain (.). When Exchange 2000 or Exchange 2003 searches for an MX record to deliver mail, Exchange 2000 or Exchange 2003 queries the DNS server. The DNS server has no knowledge of MX records on the Internet or other DNS servers that might have those records, so Exchange 2000 or Exchange 2003 is unable to route any outbound messages.

To resolve this issue:

1. On the DNS server:
   1. Open the DNS Console.
   2. Click to expand your DNS server name, and then click to expand Forward Lookup Zones. The root zone is listed as a dot (.). If the root zone is displayed under Forward Lookup Zones, delete the root zone.
   3. Right-click the DNS server object, and then click Properties on the shortcut menu. Click the Forwarders tab, and then type the IP addresses of one or more DNS servers with Internet name resolution. This allows Exchange 2000 to properly route outbound mail.
2. Because there was a root zone, there are not any root hint servers available. Simply restart the DNS server, and it should restart configured with root hint servers. However, if you cannot restart the DNS server, you must stop and restart the DNS service. To stop and restart the DNS service on the DNS server:
   1. Click Start, click Run, type cmd, and then click OK.
   2. At the command prompt, type net stop dns, and then press the ENTER key.
   3. Type net stop netlogon, and then press the ENTER key.
   4. Type net start netlogon, and then press the ENTER key.
   5. Type net start dns, and then press the ENTER key.
3. Go to the server that is running Exchange 2000 or Exchange 2003. Use the following steps to flush the DNS cache and then re-register the DNS cache on the server that is running Exchange 2000 or Exchange 2003:
   1. Open a command prompt window.
   2. Type the following command:
      ipconfig /flushdns
   3. Type the following command:
      ipconfig /registerdns

If you do not have access to the DNS Console and cannot make changes to the configuration of your DNS server, or if you want to keep DNS resolution local to your internal network and want to retain the forward lookup zone for the root (.) domain, use the following procedure to work around this issue:

1. Go to the server that is running Exchange 2000 or Exchange 2003.
2. Open the properties of the SMTP virtual server.
3. Click the Delivery tab, click Advanced, and then click Configure to configure external DNS servers. Specify the address of one or more DNS servers with external name resolution.

After you perform these steps, Exchange 2000 or Exchange 2003 should query the specified DNS servers to route outbound mail through this SMTP virtual server. If you have more than one server that is running Exchange 2000 or Exchange 2003, you must follow this process on every server that is running Exchange 2000 or Exchange 2003.

For more information about the root zone in Windows 2000, click the following article number to view the article in the Microsoft Knowledge Base: