Transport troubleshooting...

This article looks good. It’s just been re-released and consolidates several other articles:

General troubleshooting for transport issues in Exchange 2000 Server and in Exchange Server 2003 https://support.microsoft.com/kb/257265

The article doesn’t mention ExTRA though. You can use the ‘Microsoft Exchange Troubleshooting Assistant’ to troubleshoot mailflow. It’s available for download here; https://go.microsoft.com/fwlink/?LinkId=55884