Enable Pipeline Tracing

Setting the value for the PipelineTracingSenderAddress parameter to "<>" captures all e-mail server-generated messages received by the Hub Transport or Edge Transport server that you are configuring. Depending on the amount of e-mail server-generated messages that your organization receives, this may place a significant load on the server and may quickly consume available disk space. Always monitor available disk space when pipeline tracing is enabled.

How to:
http://technet.microsoft.com/en-us/library/bb125018.aspx