This is a response from ASH at stackoverflow:
There are so many ways to do this!!
SMTP logging is diabled by default. You can read about it and how to enable it at these links.
Understanding Protocol Logging: http://technet.microsoft.com/en-us/library/aa997624.aspx
Configuring Protocol Logging: http://technet.microsoft.com/en-us/library/bb124531.aspx
Set-ReceiveConnector "My Receive Connector Name" -ProtocolLoggingLevel Verbose
Set-SendConnector "My Send Connector Name" -ProtocolLoggingLevel Verbose
And could see the logs on
C:\Program Files\Microsoft\Exchange Server\V14\TransportRoles\Logs\ProtocolLog