IMAP Overview, History, Versions and Standards
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The Post Office Protocol (POP, or POP3 after its current version) has become the most popular protocol for accessing TCP/IP mailboxes not because of its rich functionality, but in spite of its lack of it. POP implements the offline mail access model, where mail is retrieved and then deleted from the server where the mailbox resides, so it can be used on a local machine. Millions of people use POP3 every day to access incoming mail. Unfortunately, due to the way the offline access model works, POP3 cannot really be used for much else.
As I said in my overview of mailbox access models, the online model is the one we would use in an ideal world, where all of us were always connected to the Internet all the time. Offline access is a necessity because this is not the case; most user client machines are only connected to the Internet periodically. The transfer of mail from the server to a client machine removes the requirement that we be online to perform mail functions, but costs us the benefits of central mail storage on the server.
You may find it counterintuitive that it could be better to have mail stored on some remote server rather than on our local computer. The main reason is flexibility of access. One of the biggest problems with offline access using POP3 is that mail is transferred permanently from a central server to one client machine. This is fine so long as an individual only uses that one machine, but what if the person has separate work and home computers, or travels a great deal? How about if a mailbox is shared by many users? These concerns have become more and more important in recent years.
Another issue is data security and safety. Mail servers run by Internet service providers are usually located in professionally-managed data centers. They are carefully controlled and monitored, and backups are done on a routine basis. Most people do not take this sort of care with their own PCs and Macintoshes, nor do they back up their data routinely. It is thus more likely that mail be lost when it is on a local machine than when it remains on the server.
Of course, we still have the problem of not wanting to force users to be online all the time to access their mail. The solution is the disconnected mailbox access model, which marries the benefits of online and offline access. Mail is retrieved for local use as in the offline model, so the user does not have to be connected to the server continuously. However, changes made to the mailbox are synchronized between the client and the server. The mail remains on the server where it can be accessed from a different client in the future, and the server acts as a permanent home base for the user's mail.
Recognizing these benefits, some attempts were made to implement POP using the disconnected access model. Typically, this was done by using POP commands to retrieve mail but still leave it on the server, which is an option in many client programs. This works, but only to a limited extent; for example, keeping track of which messages are new or old becomes an issue when they are both retrieved and left on the server. POP simply lacks the features required for proper disconnected access, because it was not designed for it.
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