You can still enable logging on your mail services. Log in to configuration centre. Stop the outbound mail service. Click on edit next to the service and change the log severity to trace. Ok the change and start the service.
You should then be able to see full logging of the service in event viewer.
Hope that's a start.
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Write a query against system/message recipients and put all the attributes you want to check on there. This gives you a way of spottong the queue building up and also perhaps see some sort of pattern developing.