Windows Server 2003 contains an SMTP service. This is useful if you have applications that need to send e-mail, but don't have access to an e-mail server. For example, in a development or demo environment.
It turns out that there is a trick to installing the service.
You get to the service through Add Remove Windows Components. It is in Application Server->Internet Information Services (IIS) -> SMTP Service. Once you start the install process, the OS will tell you to insert your Windows Server 2003 disc. It will then tell you that it is unable to find "adsiisex.dll".
It turns out that this DLL is in the "IMS.cab" file in the i386 directory. There is no easy way to get the browse dialog to find the dll inside of the cab file. So, you need to manually extract all of the files from IMS.cab to a folder on your hard drive. You then point the browse dialog to that folder, and the install will complete successfully.
This applies to both Windows Server 2003 and Windows Server 2003 R2.