1) Start Internet Information Services Manager or open the Internet Information Services (IIS) snap-in.
2) Expand Server_name, where Server_name is the name of the server, right-click Default SMTP Virtual Server, and then click Properties.
3) Click the Access tab, and then under Access control, click Authentication.
4) Click to select either or both the Basic authentication and the Integrated Windows authentication check boxes, click to clear the Anonymous access check box (if it is selected), and then click OK.
By doing so, authentication is required before access is granted to the SMTP virtual server. In this case, if the user or computer does not successfully authenticate, the user or computer cannot send mail to the server.