If you have the SMTP feature installed (an SMTP server that works through IIS), logging is not enabled by default. To enable logging, perform the following steps:

Click on Start > Administrative Tools > Internet Information Services (IIS) 6.0 Manager. Expand your SMTP server and right click on your SMTP Virtual Server. Choose Properties:

Check "Enable logging" and click on Properties:

The log file directory is where the logs will be stored. In this case, it is C:\Windows\System32\LogFiles\SmtpSvc1\exyymmdd.log. Click on the Advanced tab to configure logging:

For the most detailed logging, check everything and click ok:

Logging is now enabled.