gives a list of KB articles explaining MTA basic concepts and troubleshooting.
For the description of MTA diagnostics logging options see ME153188
"Reason code unable-to-transfer and diagnostic code loop-detected" - If it's not a single server environment, try the following suggestions below:
1) Make sure that there are no connectivity issues between the Exchange 5.5 server. Check DNS, HOSTS and LMHOSTS entries on the server and verify that those files (HOSTS and LMHOSTS) has correct entries pointing to the correct servers.
2) Review the GWART0.DAT and GWART1.DAT files from the Exchange Server in the Sending Site. This should indicate the route(s) mail intended for Destination Site should take. Then you can further review the GWART files within the sites the mail passes through en route to the Destination Site.
For example, the following are routing entries from a GWART*.DAT file from a Sending Site. Specifically, the entry below shows the route mail should take to reach the intended Destination Site, where the Destination Site's Directory Name is BELLEVUE. In this example, the Intermediary or Hub Site is named BELLINGHAM:
X400 TO BELLINGHAM, 0002
/O=FWWB/OU=BELLEVUE OFFICE/../CN=X400 TO BELLINGHAM
/O=FWWB/OU=BELLINGHAM/../CN=X400 TO BELLEVUE
If the information is wrong, try the following:
Run DS/IS consistency checker
Recalculate routing on the MTA
Recalculate routing on the site addressing
Turn logging on the MTA (Then look at the application log again)
Stop and start the services.
Also see Microsoft article "Size Limits May Cause Message Looping" - ME160534
See also the "MTA Troubleshooting Guide" link.
Reason code: unable-to-transfer, unrecognised-OR-name