To prevent the Exchange 2000/2003 server from relaying e-mail messages:

1.    Start the "Exchange System Manager".

2.     browse to organization_name object, and then expand the Servers node, server_name object of the server on which you want to prevent mail relay, and then expand the Protocols node.

3.    Expand the SMTP node, right-click the virtual SMTP server on the appropriate server, and then click Properties.

4.    Click the Access tab, and then click Authentication.

5.    Click to select either the Basic Authentication check box, or the Windows security package check box, or both of these check boxes, and then click to clear the Anonymous access check box. When you select the Basic Authentication check box, you need to provide a default user domain. Click OK.

6.    If  the "Anonymous access" check box selected and you don’t select any other check box on this page, all of the users and computers can gain access to the Exchange 2000/2003 SMTP server. This setting disables inbound authentication.

7.    If you click to select either the Basic Authentication check box, or the Windows security package check box, or both of these check boxes, and you click to clear the Anonymous access check box, authentication is required to gain access to the Exchange 2000/2003 SMTP server. If the user or computer does not successfully authenticate, the user or computer cannot send mail to the server.

8.    Click Relay.

9.    In the Relay Restriction dialog box, several options are available. The only the list below option is enabled by default; the list below this option is empty. The Allow all computers which successfully authenticate to relay, regardless of the list above option is also enabled by default, which allows users and computers that can authenticate with the server to relay through the server. This option allows the Exchange 2000/2003 server to relay mail from your internal network clients. Note that if you allow only anonymous access, the server cannot authenticate users or computers.

10. Click Add. You can allow a single computer, a group of computers, or an entire domain to relay through the server by making the appropriate selection in the Computer dialog box.

11. Allowing access by IP address or domain name is helpful for users who do not authenticate with the Exchange server .Click Cancel if you do not want to make any changes.

12. In the Relay Restrictions dialog box, click OK.

13. Click Apply, and then click OK in the Default SMTP Virtual Server Properties dialog box.