Mail may not be delivered to certain domains if Server OS is Windows Server 2003

When a user in your organization tries to send messages to certain domains by using Simple Mail Transfer Protocol (SMTP), those messages are not delivered successfully. This issue can occur with either IIS or Exchange when the underlying Operating System on the server is Windows Server 2003.

Additionally, an event that is similar to the following appears in the event log:

Event ID: 4000 Source: SMTPSVC
Event Type: Warning
Description: Message delivery to the remote domain "contoso.com" failed. An internal DNS error caused a failure to find the remote server. 0000: c00402e7

This issue occurs if all the following conditions are true:

• The DNS computer that your SMTP computer queries to obtain the mail exchanger (MX) resource records of the destination computer is configured to only accept User Datagram Protocol (UDP) queries.
• The Windows Server 2003-based SMTP computer is configured to use UDP first and then to fail over to Transmission Control Protocol (TCP) if the SMTP computer receives a truncated name resolution response.
• The DNS computer that your SMTP computer queries returns a truncated UDP response.
  
  Note A UDP response is truncated if there are too many MX resource records to fit in a single UDP packet.

In this situation, the name lookup operation from your SMTP computer is unsuccessful because the DNS computer does not respond to the TCP query. In this situation, you may experience the following symptoms:

1. The SMTP computer submits a query to the DNS computer by using UDP.
2. The DNS computer returns a truncated response to the SMTP computer.
3. Because the SMTP computer is configured to fail over to TCP if the SMTP computer receives a truncated response, the SMTP computer discards the UDP response, fails over to TCP, and then resubmits the DNS query to the DNS computer by using TCP.
4. Because the DNS computer is configured to only respond to UDP queries, the TCP query from the SMTP computer is unsuccessful.

Therefore, the message cannot be delivered to the destination domain.

Warning If you edit the metabase incorrectly, you can cause serious problems that may require you to reinstall any product that uses the metabase. Microsoft cannot guarantee that problems that result if you incorrectly edit the metabase can be solved. Edit the metabase at your own risk.

Note Always back up the metabase before you edit it.

To resolve this issue, configure the Simple Mail Transfer Protocol (SMTP) service on the Windows Server 2003 computer to only use UDP. When you configure the Simple Mail Transfer Protocol (SMTP) service to use UDP only, the SMTP computer uses the results from a truncated response that the SMTP computer receives from a DNS computer to complete the message delivery to the destination domain. To configure the Simple Mail Transfer Protocol (SMTP) service to use UDP only, add or modify the following metabase key: Note The following table describes the possible values that you can assign to key ID 36997: To modify the metabase, follow these steps:

1. Start the Microsoft Internet Information Services Manager tool. To do this, click Start, point to Administrative Tools, and then click Internet Information Services (IIS) Manager.
2. Right-click Computer name (local computer), and then click Properties.
3. Click to select the Enable Direct Metabase Edit check box, and then click OK.
4. Open the Metabase.xml file by using Notepad. To do this, click Start, click Run, type notepad %systemroot%\system32\inetsrv\metabase.xml, and then click OK.
5. If the following lines of code are not present, paste them under the <IIsConfigObject Location ="/LM/SmtpSvc" heading:

   <Custom
   		Name="SmtpUseTcpDns"
   		ID="36997"
   		Value="2"
   		Type="DWORD"
   		UserType="IIS_MD_UT_SERVER"
   		Attributes="INHERIT"
   	/> 

   Note Paste these lines of code in numeric order based on the ID number 36997. For example, paste these lines of code after the lines of code that contain ID number 36931 and before the lines of code that contain ID number 49875.
6. If this code section already exists, modify the Value item to a value of 2.
7. Save your changes to the Metabase.xml file, and then quit Notepad.

For more information, click the following article number to view the article in the Microsoft Knowledge Base: