Microsoft Knowledge Base Article 310336 contains the following summary:

This article describes how to configure a remote domain for an Internet Information Services (IIS) 5.0 Simple Mail Transport Protocol (SMTP) mail relay server.

IIS 5.0 in Windows 2000 includes a full-featured SMTP server that you can use to route mail on your internal network. The SMTP server can also accept mail from the Internet. Although the IIS 5.0 SMTP service does not include an easily accessible mechanism to retrieve mail, it can play a vital role in accepting mail from the Internet and relaying it to your SMTP/POP3 server (such as Microsoft Exchange 2000 Server). You can configure the IIS 5.0 SMTP server to accept mail for the domain to which the computer belongs, as well as for other domains.

A standalone server that is running the IIS 5.0 SMTP service provides a good measure of security because it acts as a layer of separation between the corporate mail server and inbound connections from the Internet. A standalone Windows 2000-based server isolates the server from the Active Directory security boundaries of the internal network. You can configure the standalone SMTP server to use the corporate mail server as a smart host. You can then configure the standalone IIS 5.0 SMTP server to relay mail that is directed only to the remote domain. If you do this, all other mail is not relayed and is rejected.