Topic Last Modified: 2007-05-10

The Microsoft Exchange Analyzer Tool performs a Domain Name System (DNS) lookup to retrieve the Mail Exchanger (MX) records for a domain. If the Exchange Analyzer cannot retrieve MX records for a domain from the DNS server, the Exchange Analyzer then tries a DNS lookup to retrieve the Host (A) records of the domains that the Exchange server is trying to send messages to.

The Exchange Analyzer displays an error if the following conditions are true:

• Exchange Analyzer cannot retrieve MX records for a domain.
  
  
• The DNS lookup operation fails to return an IP address for any one of the Host (A) records.
  
  

This error indicates that the DNS Mail Exchanger (MX) and Host (A) records appear to be missing for the domain and servers. This error may cause messages destined for remote domains to back up in the Exchange server Remote Delivery queues in addition to other routing or service delays.

To determine mail hosts, the sending Exchange server checks for an MX record. Next, the sending server resolves the MX record to an IP address by checking for an address record (A record).

The DNS lookup of the Mail Exchanger (MX) and Host (A) records can fail for the following reasons:

• The DNS server is down or unreachable because of a network failure or other reasons.
  
  
• The Host (A) record for the remote server is missing from the DNS server because the record is unavailable or incorrect.
  
  
• The DNS server is not configured to allow dynamic host record updates.
  
  
• The system is trying to send non-delivery reports (NDRs) in response to spam messages that may disguise their originating domains. If this is the case, you should delete those messages. See the link for the Exchange Server Badmail Deletion and Archiving tool in the For More Information section of this article.
  
  

To verify that the DNS server is online and that the MX and Host records are present

1. Verify that the DNS server is running by doing one or more of the following checks:

   • Look at the DNS server status from the DNS Administration program on the DNS server.
     
     
   • Restart the DNS server. For more information, see "Start, stop, pause, or restart a DNS server" (http://go.microsoft.com/fwlink/?LinkId=62999).
     
     
   • Verify the DNS server responsiveness by using the nslookup command. For more information, see the instructions in "Verify DNS server responsiveness using the nslookup command" (http://go.microsoft.com/fwlink/?LinkId=63000).
     
     

2. Use nslookup to verify that the Mail Exchanger (MX) records are configured correctly. For more information, see "How to Use Nslookup to Verify MX record configuration" in the "Verifying DNS Design and Configuration" section of the Exchange Server Transport and Routing Guide (http://go.microsoft.com/fwlink/?LinkId=47579).

3. 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" (http://go.microsoft.com/fwlink/?LinkId=63001).

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

   • Manually add an A resource record. For more information, see "Add a resource record to a DNS zone" (http://go.microsoft.com/fwlink/?LinkId=63002).
     
     
   • Manually modify an A resource record. See the instructions in "Modify an existing resource record" (http://go.microsoft.com/fwlink/?LinkId=63014).
     
     
   • Configure the DNS server to enable dynamic host record updates. For more information, see Microsoft Knowledge Base article 816592, "How to configure DNS dynamic updates in Windows Server 2003" (http://go.microsoft.com/fwlink/?linkid=3052&kbid=816592).
     
     

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