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
\Exchange Server\TransportRoles\Logs\ProtocolLog\SmtpReceive
\Exchange Server\TransportRoles\Logs\ProtocolLog\SmtpSend