Microsoft Knowledge Base Article 308161 contains the following summary:

This step-by-step article describes how to set up a Simple Mail Transfer Protocol (SMTP) virtual server on a Windows 2000-based computer.

Microsoft SMTP Service uses the Internet-standard SMTP to transport and deliver mail messages. Microsoft SMTP Service does not support the use of individual mailboxes; it places incoming messages in a Drop folder or it forwards them to other SMTP servers (if the message is addressed to a remote domain). This feature allows Microsoft SMTP Service to be used as a mail receiver for other programs.