Exchange Protocol Logging is not Enabled by Default in Exchange 2007. We have to enable the logging if required. Below powershell commands help you to enable the
Set-SendConnector “Send Connector Name” -ProtocolLoggingLevel verbose
Set-ReceiveConnector “Connector Name” -ProtocolLoggingLevel verbose
You can also enable the connector through Exchange Management Console.
EMC -> Exchange Orginization -> Hub Transport Servers -> Send connectors (tab) -> Right click on the required connector properties -> select Verbose
Location of STMP Logs in the below mentioned location