Additionally, an event that is similar to the following appears in the event log:
- 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.
- The SMTP computer submits a query to the DNS computer by using UDP.
- The DNS computer returns a truncated response to the SMTP computer.
- 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.
- Because the DNS computer is configured to only respond to UDP queries, the TCP query from the SMTP computer is unsuccessful.
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:
Key type: DWORD
Key value: 2
|0||DNS queries use UDP and then fail over to TCP upon receipt of a truncated response|
|1||DNS queries use TCP only|
|2||DNS queries use UDP only|
- 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.
- Right-click Computer name (local computer), and then click Properties.
- Click to select the Enable Direct Metabase Edit check box, and then click OK.
- 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.
- If the following lines of code are not present, paste them under the <IIsConfigObject Location ="/LM/SmtpSvc" heading: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.
- If this code section already exists, modify the Value item to a value of 2.
- Save your changes to the Metabase.xml file, and then quit Notepad.