Topic Last Modified: 2006-04-13

The Microsoft® Exchange Server Analyzer Tool performs a Domain Name System (DNS) lookup on the DNS server to retrieve the Host (A) record of the Exchange server. If the DNS lookup operation does not return an IP address, the Exchange Server Analyzer displays a warning that the Host (A) record appears to be missing.

The DNS lookup of the Host (A) record from the Exchange Server 2003 computer could fail for the following reasons:

When the DNS server is down, there can be mail routing delays and other service failures.

Exchange Server 2003 relies on host name resolution for both local and external SMTP-based messaging systems in the network. Host name resolution is based primarily on DNS, and DNS is a critical network service. The failure of a DNS server can prevent users from locating your messaging system. Active Directory and the Exchange Server 2003 organization cannot function without DNS. Messaging clients will query DNS to locate internal resources, such as domain controllers and mailbox servers. Exchange Server 2003, on the other hand, relies on DNS to retrieve Internet Protocol (IP) addresses of domain controllers for directory lookups and external Simple Mail Transfer Protocol (SMTP) hosts when Exchange Server 2003sends messages to the Internet.

To fix the issue, verify that the DNS server is running and that the Host (A) record is registered.

To verify that the DNS server is online and that the Host record is present
  1. Verify that the DNS server is running by performing one or more of the following steps:

  2. Use nslookup to verify that the Host (A) record exists on the DNS server. For more information, see "To verify A resource records exist in DNS" (

  3. If the Host (A) resource record does not exist, manually add or modify the A resource record or configure the DNS server to allow dynamic record updates

For information about Exchange Server 2003 system monitoring, see "System Monitoring with Exchange Server 2003 Management Pack" in the Exchange Server 2003 Management Pack Guide for MOM 2000 SP1 (