Horizontal Scalability is the ability to expand the capacity of a SIMS environment by adding more servers. Message Access Proxy servers make horizontal scalability possible by having clients point to a single host name which provides access to their mail. Proxies do the work of routing the protocol traffic to and from the appropriate Message Store server. Since proxies allow clients to access their mail folders through a host name which is independent of the actual message store host name, capacity can be added without any burden or reconfiguration on the clients. (For example, having to reconfigure the message access server on each client.)
A single SIMS server cannot support the hundreds of thousands, or even millions of users that Internet Service Providers (ISPs) need to support. 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. Mail requests are routed through the system and sent to a proxy server via round-robin DNS (DNS that can return more than one IP address in round-robin fashion to distribute load among multiple proxy servers). 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.