To provide basic Simple Mail Transfer Protocol (SMTP), POP3, and IMAP4 access to remote clients, create a publishing rule for your Exchange server, and create an access rule that allows SMTP traffic to travel from the internal network to the external network.
To create an Exchange publishing rule, follow these steps:
In the ISA Server Management console tree, expand your ISA Server node, and then click Firewall Policy.
In the Tasks pane, on the Tasks tab, click Publish A Mail Server.
On the Welcome To The New Mail Server Publishing Wizard page, type in the rule name (such as Mail Server Publishing Rule), and then click Next.
On the Select Access Type page, select Server-To-Server Communication: SMTP, NNTP, and then click Next.
On the Select Services page, select the protocols you want to publish. SMTP will be necessary to send mail, and POP3 mail might be necessary if your clients use external POP3 accounts. If you want to publish a news server, you can also select NNTP.
On the Select Server page, type in the IP address of your Exchange server, and then click Next.
On the IP Addresses page, select the networks on which you want to publish the Exchange server. Most often you will select External and VPN Clients. Click Next.
On the Completing The New Mail Server Publishing Rule Wizard page, click Finish.
To create an SMTP traffic access rule, follow these steps:
On the Tasks tab, click Create New Access Rule.
On the Welcome To the New Access Rule Wizard page, type in the name of your access rule (like SMTP to External).
On the Rule Action page, select Allow, and then click Next.
On the Protocols page, in the This Rule Applies To drop-down list box, select Selected Protocols and then click Add.
In the Add Protocols dialog box, expand the Mail container, select SMTP, POP3, or NNTP (depending on what you want to publish), click Add, and then click Close. On the Protocols page, click Next.
On the Access Rule Sources page, click Add to open the Add Network Entities dialog box. Expand the Networks container, select Internal, click Add, and then click Close. Click Next.
If you want to limit the computers that can send mail using port 25, create a computer set with only the Exchange server and those other clients that have a specific need. Replace the internal network described earlier with the computer set you created.
On the Access Rule Destinations page, click Add to open the Add Network Entities dialog box. Expand the Networks container, select External, click Add, and then click Close. Click Next.
On the User Sets page, select the users that will have rights to send SMTP. Typically, you will choose Domain Users. Click Next.
On the Completing The New Access Rule Wizard page, click Finish.
Click Apply, and then click OK to commit the changes.
You can control SMTP traffic on your network by using the SMTP Message Screener, which can filter SMTP traffic based on your criteria, and then delete, hold, or forward messages. For more information on how to configure the SMTP Message Screener, see the ISA Server Help file.