Multiple servers are needed to support such populations. Without proxy servers, each user would have to specify their server host name to retrieve mail. By using proxy servers, messages can be accessed through one virtual mail server, while any number of actual mail servers perform actual message storage and retrieval. In the figure above, users log in to the system using the domain name, mail.isp.net . The message access proxy authenticates the user through a replicated LDAP directory, then sends the request to the appropriate message access server.
Additional capacity is achieved by adding more message access servers behind the proxies.
This deployment allows for easy expansion of capacity and by virtue of round-robin DNS allows mail access proxies to be treated as field replaceable units. If mail.isp.net needs to expand message store capacity to accommodate new customers they can do so either by expanding the capacity of an existing Message Store server by adding system resources or they can add a new Message Store server. In either case, clients will not be required to change their mail server hostname setting.