XADM: Mail May Not Be Delivered When DNS Resolution Uses Only the UDP Protocol

When you configure your DNS servers to accept name resolution requests by using only the User Datagram Protocol (UDP), mail to domains whose name resolutions return a truncated response may not be delivered.

This issue occurs if the Simple Mail Transfer Protocol (SMTP) server is configured to try UDP first, and to then failover to Transmission Control Protocol (TCP) after it receives a truncated name resolution response.

When the SMTP server receives the truncated response over UDP, it tries to use TCP to resolve the domain names. However, because the DNS servers are configured to not support TCP, mail delivery is not successful.

NOTE: When this issue occurs, the SMTP service logs an event, which states that it failed to connect to DNS over TCP. Because the administrator is aware that the DNS servers do not support TCP in this case, the administrator will be alerted to the cause of this behavior.

To work around this issue, reconfigure the SMTP service to use only UDP. To do this, edit the Microsoft Internet Information Services (IIS) metabase. To use the MetaEdit utility to edit the IIS metabase, follow these steps.

WARNING

• Microsoft provides this utility "as-is," without the warranty either expressed or implied, including, but not limited to, the implied warranties of merchantability and or fitness for a particular purpose. This article assumes that you are familiar with the IIS metabase and the tools that are used to back up and to restore the metabase. Microsoft support professionals can only help explain the functionality of the IIS snap-in for the Microsoft Management Console (MMC).
• Using the MetaEdit utility incorrectly causes serious problems that require you to re-install IIS or your operating system. Microsoft cannot guarantee that problems that result from the incorrect use of the MetaEdit utility can be solved. Use MetaEdit at your own risk.
• For information about how to edit the IIS metabase, view the Help topics in MetaEdit.
  
  NOTE: Always back up the metabase by using the IIS snap-in for the MMC before you edit the metabase. Verify that the backup copy can be restored if a problem occurs. For more information, view the "Configuration Backup/Restore" Help topic in the MMC.

1. Obtain and install the MetaEdit utility.For additional information about how to obtain and install the MetaEdit utility, click the article number below to view the article in the Microsoft Knowledge Base:
   232068 HOW TO: Download, Install, and Remove the IIS MetaEdit 2.2 Utility
2. Perform a backup of the IIS metabase. To do this, follow these steps:
   1. Start the IIS snap-in. To do this, click Start, point to Programs, point to Administrative Tools, and then click Internet Services Manager.
   2. Right-click the server object, and then click Backup/Restore Configuration.
   3. Click Create backup, type a descriptive name for the backup, click OK, and then click Close.
3. Start MetaEdit 2.2.
4. Expand LM, and then click SmtpSvc.
5. In the right pane of the Metabase Editor, search the Id column for a DWORD value with the Id of 36997. Right-click 36997 (if it is listed), and then click Modify. If this DWORD value is not listed, create it. To do this:
   1. Click SmtpSvc in the left pane of the Metabase Editor (if it is not already selected).
   2. On the Edit menu, point to New, and then click DWORD.
   3. In the box that is next to the Id list, type 36997.
6. In the Edit Metabase Data dialog box, type 2 in the Data box, and then click OK.
   
   The following list describes the options for the 36997 DWORD value:

   Value:       Description:
   
   0x0          UDP followed by TCP upon truncation
   
   0x1          TCP only
   
   0x2          UDP only
   						

7. Quit the Metabase Editor.