The server listens to all IP addresses and presents a customized interface before the authentication occurs. The server does this by looking at the URL and by determining if it contains a known domain and presents the per domain Login screen for the domain.
For example, for the per domain Login screen: http://webmail.sesta.com/, the server presents the page from the location: <uwc-deployed-dir>/webmail/<domain-name>/en/default.html.
In this case a user does not have to suffix @domain to the user name to login.
For more information on Customizing the Mail Component, Sun Java System Messenger Express Customization Guide.