Você pode ter que esperar até que o processo libere seu buffer de log e grave no arquivo de log. Eu sei que para o serviço HTTP você pode limpar manualmente o buffer e forçá-lo a gravar no arquivo usando este:
netsh http flush logbuffer
Estou procurando algo equivalente ao do serviço SMTP.
EDITAR:
Ou dê uma olhada no presente no blog de Steve Schofield :
I was working on installing Windows Server 2008 x64 edition. I discovered the SMTP Service wasn't logging. SMTP was working and emails were going out. My install is 'custom' and installs just the modules we needed. Turns out, there is a small dependency on the ODBC logging module so the SMTP service logging would work. More importantly iislog.dll. Here are the instructions to fix and reproduce the behavior.
To correct it. I'm assuming you have the SMTP Service already installed and it's not logging.
1) Install ODBC Logging module (role service in Server Manager)
2) Stop / Start the SMTP Service
3) Verify your SMTP service is configured for logging. It's not on by default.
4) Try a local telnet test (assuming the telnet client is installed)
5) Look at your log folder.