A proxy message access server differs from a typically configured SIMS server in that instead of serving the POP/IMAP requests itself, it forwards to the request to the message access server with the requested mail folders (FIGURE A-1). The proxy may or may not have a local message store, but it acts as a virtual message access server by forwarding POP/IMAP requests to the appropriate message access server. Message access proxies are useful for a number of reasons such as horizontal scalability and internet access to private intranet mail systems. These are discussed in "Proxy Server Models" on page 286.
Proxies accept client POP/IMAP requests for mailbox access and authenticates requestor's passwords. The proxy's mail access daemon (imaccessd) forwards the request to the appropriate mail access server (i.e., the server containing the desired mailbox). The requestor is again authenticated and the mail access daemon on the server accesses the requested mailbox. At this point, the proxy acts as a simple pipe between the client and the mail server, forwarding whatever one sends to the other, until either the client or the server closes the connection.
Although a proxy server may not have the requested message store, from the client's point of view, the proxy acts like the requested mail access server. Because the proxy communicates with the requested mail server using POP/IMAP, from the requested mail access server's point of view the proxy appears as another mail client.
SIMS message access proxies have two configurations, a pure proxy, which acts only as a proxy for SIMS mail servers, and a message access proxy/message access server which can act as a proxy for some mail addresses and as a full mail server for local addresses. Note that a pure proxy does not have an IMTA installed, and only supports POP and IMAP but not SMTP. An example of these is shown in FIGURE A-1.
FIGURE A-1 Pure Proxy Server and Mail Access Proxy/Mail Access Server