Topic Last Modified: 2007-03-16

The Microsoft Exchange Analyzer tool queries the Active Directory directory service to determine the value for the msExchSmtpExternalDNSServers attribute in the protocolCfgSMTPServer class for each Exchange Server object. The value of the msExchSmtpExternalDNSServers attribute, if set, specifies the address of the external Domain Name System (DNS) server that the Simple Mail Transfer Protocol (SMTP) Virtual Server Instance uses.

After retrieving the address of the primary external DNS server, the Exchange Server Analyzer opens a TCP socket connection to port 53 on the server. This connection is made by using a custom object processor that returns a specific string if the connection is successful. If the Exchange Server Analyzer does not receive 53 Available as part of the returned string, a warning is displayed.

This warning indicates that the server did not respond to a connection attempt on TCP port 53. By default, DNS servers listen on TCP socket 53 for communications such as name resolution queries.

SMTP is the native mail protocol for mail submission and mail transport for Exchange Server. This means that clients use SMTP to send messages and Exchange servers use SMTP to deliver messages and message data.

For Exchange Server to deliver an outbound internet message via the SMTP service, DNS is employed by the following method:

  1. An internal user sends a message to a recipient in a remote domain.

  2. To determine if the recipient is local or remote, the SMTP virtual server on the sender's Exchange server uses internal transport functions to query the global catalog server for the recipient address. If the recipient address on the message is not in a recipient policy, it is not stored in Active Directory. Therefore, Exchange determines that the message is destined for a remote domain.

  3. If necessary, the Exchange server delivers the message to the appropriate SMTP virtual server.

  4. The SMTP virtual server uses its IIS metabase information to determine the method for delivering a message to a remote domain.

  5. The SMTP virtual server on the Exchange server then performs one of two actions:

    • Uses DNS to look up the IP address for the target domain, and then attempts to deliver the message.

    • Forwards the message to a smart host that assumes responsibility for the DNS resolution and delivery of the message.

If the primary external DNS server is unreachable and you are running Exchange 2000 Server, mail delivery will be slow. A supported hotfix is available from Microsoft to correct the problem of message delivery to external recipients being slower than expected when the primary external DNS server is not available. Contact Microsoft Product Support Services to obtain this hotfix.

Additionally, you will want to troubleshoot why the primary external DNS server is not available, or change the DNS settings on the Exchange server to use a different DNS server for name resolution.

To correct this warning, do the following:
  1. Make sure that the external DNS server has been started, and that there is connectivity to it from the Exchange network.

  2. Use the PING command to determine whether the external DNS server is reachable.

For more information about slow mail delivery when the primary DNS server is not available, as well as how to obtain a hotfix for this issue, see the Microsoft Knowledge Base article 829722, "Mail delivery is slow in Exchange 2000 when the primary DNS server is not available" (

For more information about DNS and Exchange Server, see the following Knowledge Base articles and WebCast: