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The most prominent security challenge related to e-mail is the number of unwanted and malicious e-mails that are sent across the Internet. These e-mails can be grouped in two categories: unwanted junk e-mail that consumes computer resources and user time but does not harm the computer, and malicious e-mails that contain viruses, worms, or Trojan horse programs. Unwanted E-Mail It has been estimated that unwanted e-mail messages consume more than 50 percent of total bandwidth usage on the Internet today. Unwanted e-mail leads to the following problems:

Wasted bandwidth on both internal and Internet networks, which may lead to increased Internet bandwidth cost, and increased nonproductive traffic on the corporate network Increased resource usage, including disk space, processor, and memory use on e-mail servers Decreased employee productivity due to reading and deleting unwanted e-mail Increased administrative costs as network administrators attempt to reduce the negative effects of unwanted e-mail Increased exposure to legal liability to users who may view offensive unwanted e-mail messages

Most organizations attempt to filter unwanted e-mail before it gets to the e-mail servers. The challenge is to determine which e-mail messages are unwanted, yet allow valid

e-mail messages into your mail systems. To filter junk e-mail, you must use an application-layer filter. These filters can inspect the SMTP messages transporting unwanted e-mail messages and filter out messages based on factors such as source e-mail address, keywords in the subject line or message body, and attachment name or extension. Malicious E-Mail Viruses and worms sent by e-mail can cause a tremendous amount of damage to corporate networks. Viruses and worm attacks are responsible for the following:

Destruction of data on servers and workstations DoS attacks on servers and workstations Lost employee productivity because a workstation or network server is unavailable Distribution of corporate secrets by means of mass-mailing worms Increased administrative costs due to repairing damaged workstations and servers Increased bandwidth use on the corporate network and Internet connection secondary to mass-mailing worms and DoS attacks

Viruses and worms most commonly access an organization s network through e-mail. Virus writers realize that e-mail is a critical service in most organizations and they exploit this fact by crafting viruses and worms that spread by e-mail. When a user opens an e-mail attachment that contains dangerous code, the code is released to the user s computer and then spreads to the rest of the network. A single infected host can damage virtually every networked device in a short period. Effective prevention of viruses and worms requires organizations to deal with the viruses at multiple levels. Antivirus programs should be installed on the e-mail servers as well as on the client computers. Another level in the defense is to block all e-mail attachments at the perimeter. An SMTP application filter on the network perimeter can then examine all e-mail attachments and scan the attachment for viruses, worms, and other dangerous code. Based on the results of the scan and its rule configuration, the application filter can delete the message, quarantine the message for further inspection, or forward the message to the e-mail server.

ISA Server provides three components for securing SMTP traffic. The first is the Mail Server Wizard, which can be used to publish the SMTP server to the Internet. The second component is the SMTP Message Screener, which can help reduce the amount of unwanted e-mail entering the organization. The third component is the SMTP application filter, which can be used to block buffer-overflow attacks or SMTP commandbased attacks on Exchange Server.

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You can use the Mail Server Wizard to make Exchange Server computers available to Internet clients. The Mail Server Wizard includes several options, one of which is publishing an SMTP server. When publishing an Exchange Server computer as an SMTP server, you create a server publishing rule that accepts SMTP traffic on the ISA Server computer s external interface and forwards the packets to the Exchange Server computer.

The Mail Server Wizard can be used to configure all types of access to your organization s Exchange Server computers. When you run the wizard, you choose the access required based on server or client type and the Mail Server Wizard configures a Web or server publishing rule with the required protocols and settings.

To configure ISA Server to publish an Exchange Server computer as an SMTP Server computer, complete the following procedure: 1. In the ISA Server Management Console, click Firewall Policy. On the Tasks tab, select Publish a Mail Server. 2. On the Welcome To The New Mail Server Publishing Wizard page, type a name for the publishing rule, and then click Next. 3. On the Select Access Type page, shown in Figure 9-1, click Server-To-Server Communication: SMTP, NNTP, and then click Next.