Why IMAP?
The short answer is that IMAP solves many of the problems inherent in legacy mail systems. Since the earliest days of networking, the best and the brightest in the computer field have looked for successively better ways to corral individual correspondence into threads of dialog that could easily be used online. This section puts IMAP in the context of several legacy mail models and its competing mail access protocol, POP.
Host-Based Email
Host-based email was the original model for email. If you’ve been using email for many years, you probably started by using this model. In the host-based email model, the MUA, MTA, and mailstore reside on the same physical machine. Figure 2-1 illustrates the host-based email model.
Figure 2-1. Host-based email
Host-based email is a study in single-points-of-failure. Because all functions are carried out by a single system, divisions of responsibility between different parts of the system are unclear. If one part craters, there’s a much greater risk that it’ll cause the rest of the system to crater. On a host-based email system, the actions of any of the three essential components (MUA, MTA, and mailstore) impact the performance of the others. Host-based email systems are frequently general-purpose compute servers or Internet servers as well. Most enterprise mail systems that have been around for many years began in this model, the shortcomings ...
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