A couple of suggestions for things to look for.
Is there a message in the error database? If so check source and destination connections for that errored message. Also, make sure it is not failing due to a tcl proc.
2nd, are you killing inbound replies from receiving system (killing the ACKS). If not the messages will attempt to get sent back through the threads the opposite direction.
You stated that the message was getting routed and Xlated successfully, so it shouldn’t be your existing route configuration.
Ryan Spires