Sometimes after the installation of the SMTP service on Windows Server 2003, the SMTP tree may not appear in the IIS Manager console.
Solution:
Click the “Start” button, then “Run…” and type:
regsvr32 %systemroot%\system32\inetsrv\smtpsnap.dll
Alternatively, %systemroot% can be replaced with the drive Windows Server 2003 is installed on
OR
Uninstalling and re-installing IIS including the SMTP service has also proven to fix the problem, it is, however, recommended to try the first method.