Sometimes it happens that your Hub Transport Server is not able to send outbound messages and you have to troubleshoot what's going on. There are several methods for troubleshooting SMTP:
- If users are complaining that they receive NDR's see if you can get a copy of the NDR. Most of the times it is a problem on the receiving SMTP Server and not on your sending SMTP Server;
- On the Hub Transport Server see if there are any entries that can reveal any issues on your Hub Transport Server;
- In the Exchange Management Console, open the Toolbox and open the Queue Viewer. Right here you can see what messages are stuck on the Hub Transport Server. Likewise you can open the Exchange Management Shell and enter the Get-Queue command;
- In the Exchange Control Panel, check the Delivery Reports to see if you can get more information about the messages being sent;
- Although very difficult, you can open the Message Tracking logs (typically located in "C:\Program Files\Microsoft\Exchange Server\V14\TransportRoles\Logs\MessageTracking" to find any detailed information;
- If you need more information on your own Hub Transport Server you can enable Transport Logging. In the Exchange Management Console, navigate to the Organization Configuration and select Hub Transport. Select the Send Connectors Tab and request the properties of the Send Connector. On the General Tab of the properties select Verbose at the Protocol Logging Level.
- By default, Protocol log files are stored on the Hub Transport Server in the `C:\Program Files\Microsoft\Exchange Server\V14\TransportRoles\Logs\ProtocolLog\SmtpSend` directory. Be careful with the transport logging since it writes a tremendous amount of data to the log files. Turn it of as soon as you've gathered enough information;
- Use the NSLookup utility to retrieve the MX records of the receiving domain;
- Use the PING command to see if you can resolve the appropriate servers and can reach these servers;
- Use the TELNET utility to see if you can open a connection on the receiving SMTP server on port 25. For more information on how to use the various Telnet command, check the Microsoft Technet website: http://technet.microsoft.com/en-us/library/bb123686.aspx;
Using these steps you should be able to troubleshoot most of the SMTP sending issues on your Hub Transport Server.