How to Bypass DNS Name Resolution to Test SMTP Mail Flow to Remote Domains

This article describes how to bypass DNS name resolution to test Simple Mail Transfer Protocol (SMTP) mail flow to remote domains. This test can be useful if you experience issues sending messages to all external domains or if you can send messages to some domains but not others. This test enables you to bypass DNS name resolution to test for SMTP connectivity to any remote domain.

To bypass DNS name resolution:

1. Create an SMTP Connector to the remote domain that you want to send messages to.
2. When you are local to the remote SMTP server, start a command prompt, and then run an ipconfig command to verify the Internet protocol (IP) address of the SMTP server of the remote domain.
3. Open the SMTP connector properties, click the General tab, and then type the IP address of the remote SMTP server in the Smart Host box.
   
   Note Format the address so that it is enclosed in brackets, for example "[10.10.250.1]".
4. Stop the Microsoft Exchange Server Routing Engine Service and the SMTP service.
5. Restart both services.
6. Use the Winroute utility to check for changes to routes.
7. Send test messages to the remote domain.

When you complete this procedure, Exchange completely bypasses the DNS service and delivers messages to the remote domain directly through the IP address. If mail delivery is successful, you may be experiencing issues with name resolution. Use additional tools to determine if the issue is caused by records that are returned by DNS to the requesting Exchange computer, or if the issue occurs because Exchange is not using that information correctly. Use utilities such as Nslookup to assess information that is being passed to Exchange from an internal DNS server and compare it to the DNS information that is available from any of the Internet DNS servers.

For additional information, click the article number below to view the article in the Microsoft Knowledge Base: If the DNS information is correct, you may need to gather a network trace of all traffic coming to and going from the Exchange computer to verify that the correct information is being passed to the Exchange computer. This trace information can also be used to confirm that Exchange is attempting to communicate with the IP address that DNS returns. If the attempts are unsuccessful, check all firewall and router configurations between these domains to ensure that port 25 is configured correctly to handle inbound and outbound traffic. Also, check with administrators of the remote domain to ensure that they are able to accept mail.