Secure Access by IP Address, Subnet, and Domain
To further enhance security, you can selectively include or exclude single computers, subnets, and entire domains from accessing a POP3 virtual server. By default, all computers have access until you add them to a list of restricted clients. However, you can restrict access by either listing only a few computers, or by specifying computers that don't require the POP3 service. This feature is very useful if you create multiple instances of a POP3 virtual server; for example, by restricting a computer's access to one POP3 server but allowing access to another, you can ensure that the client logs on only to the service configured for that client.
To secure access by IP address, subnet, and domain: