Microsoft Internet Security and Acceleration (ISA) Server enables you to publish internal servers to the Internet without compromising the security of your internal network. You can configure Web publishing and server publishing rules that determine which requests should be sent to a server on your local network, providing an increased layer of security for your internal servers.

The publishing feature in ISA Server secures internal server access by external clients.
For example, you can place your Microsoft Exchange 2000 server on the Intranet, protected by the ISA Server computer, and create server publishing rules that allow the e-mail server to be published to the Internet. Incoming e-mail to Exchange is intercepted by the ISA Server computer, which gives the appearance of an e-mail server to clients. ISA Server can filter the traffic and forward it on to the Exchange Server.

Your Exchange Server is never exposed directly to external users and sits in its secure environment, maintaining access to other internal network services.
A common ISA Server scenario involves securing the Simple Mail Transfer Protocol (SMTP) communication of mail servers. For example, ISA Server can protect an Exchange Server. The Mail Server Security Wizard configures the policy needed to allow communication between an Exchange Server computer and the Internet. The wizard adds a set of server publishing rules, which redirect communication from Internet users at a particular port to a specified internal Internet Protocol (IP) address. The wizard also creates protocol rules that dynamically open ports for outgoing communication.

ISA Server also includes a Remote Procedure Call (RPC) application filter, which uniquely provides an extra layer of security to the Exchange Server publishing model. This RPC application filter enables secure communication between Microsoft Outlook clients and an Exchange Server, over the Internet.

The Exchange Server that you are publishing can be installed on the ISA Server computer or on the local network. The following describes some Exchange Server publishing scenarios:

• Exchange Server on local network (recommended)
• Exchange Server on the ISA Server computer (not recommended)

ISA Server includes message-filtering functionality, which can be used to control the incoming SMTP traffic communicated through the ISA Server. In this architecture, ISA Server sends messages to the SMTP Service, which then sends it to the Message Screener, which sends the messages back to the SMTP service, which may then relay it.