Microsoft Knowledge Base Article 310155 contains the following summary:
This step-by-step article describes how to disable the Internet Information Services (IIS) 5.0 Simple Mail Transfer Protocol
(SMTP) Service socket pooling functionality.
You can use the socket pooling functionality in IIS 5.0 to host multiple sites that use a different Internet Protocol (IP) addresses but the same port number. If you use socket pooling, you can reduce the amount of resources that the IIS 5.0 services consume. IIS 5.0 can host many more sites than IIS 4.0 because of socket pooling.
If you use socket pooling, IIS 5.0 services listen on all IP addresses regardless of the IP address that you configure in the IIS snap-in.