Comparing Two Approaches to Remote Mailbox Access: IMAP vs. POP
POP was designed to support “offline” mail processing. In the offline paradigm, mail is delivered to a (usually shared) server, and a personal computer user periodically invokes a mail “client” program that connects to the server and downloads all of the pending mail to the user’s own machine. Thereafter, all mail processing is local to the client machine. Think of the offline access mode as a kind of store-and-forward service, intended to move mail (on demand) from the mail server (drop point) to a single destination machine, usually a PC or Mac. Once delivered to the PC or Mac, the messages are then deleted from the mail server. Although the limitations of offline access have triggered interest in using POP in online mode, POP simply doesn’t have some of the functionality needed for high-quality online (or disconnected) operation. Indeed, POP’s “pseudo online” mode of operation, wherein client programs leave mail on the server, often depends on pervasive availability of a remote file system protocol in order for the mail client to access or update saved-message folders or message state information such as status flags.
IMAP can also do offline processing, but its special strength is in online and disconnected operation. In online mode, mail is again delivered to a shared server, but the mail client does not copy it all at once and then delete it from the server. It’s more of an interactive client-server model, where the client can ask the server for headers, or the bodies of specified messages, or to search for messages meeting certain criteria. Messages in the mail repository can be marked with various status flags (e.g. “deleted” or “answered”) and they stay in the repository until explicitly removed by the user –which may not be until a later session. In short: IMAP is designed to permit manipulation of remote mailboxes as if they were local. Depending on the IMAP client implementation and the mail architecture desired by the system manager, the user may save messages directly on the client machine, or save them on the server, or be given the choice of doing either.
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