Some sites deploying Messenger Express might want to provide an alternative method of authenticating users. These sites might have portal service in which users sign on once at the “front door” to use various services without reauthentication. In this environment, the Messenger Express login mechanism has to recognize that the other service has already performed the authentication and recognize the user based on credentials provided by the other service.
This is known as proxy authentication. In most cases, users enter a user name and password to access the site. One of the services users can access from the site would be the Messenger Express. When users click on the link to open Messenger Express, it does not ask for a username or password again, because with proxy authentication the user name and password provided at the initial site login is referenced.