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. The message filtering functionality is implemented in two components:

• The SMTP filter is installed by default, but not enabled. It is used to specify which SMTP commands are allowed in the incoming SMTP traffic. The SMTP filter is always installed, regardless of whether you install the message screener. However, if the message screener is not installed, then the filter can screen only SMTP commands and not message content.
• The message screener is an extension to the Exchange 2000 SMTP server that can be optionally installed. During ISA Server setup, you can select to install the message screener only if an SMTP server is installed on the computer. The message screener is used to filter messages, specifying which keywords and attachments are permitted and which users and domains are denied. You configure the message screener using the SMTP filter's property sheets.

ISA Server and Exchange 2000 can be deployed on the same (local) computer or on different computers. Depending on your network configuration, you will need to set up and configure SMTP Server, ISA Server, the message screener, and the SMTP filter differently. The following describes the different deployment scenarios, and how you should configure ISA Server and Exchange Server for the different scenarios.

The Mail Server Security Wizard in ISA Server does not configure the SMTP filter when SMTP Server and ISA Server are located on the same computer. The ISA Server and the SMTP Server must be specially configured if you want the SMTP Filter functionality.

• ISA Server and Exchange 2000 deployed on same (local) computer
• ISA Server and Exchange 2000 deployed on different computers