IMAP is a superset of POP, and it can do everything that POP can (though it doesn't always do it in quite the same way). In addition, IMAP introduces a number of new features.
It's becoming increasingly more common for people to use more than one computer. One at work, another at home, and perhaps a laptop when traveling. POP doesn't lend itself well to checking mail on multiple machines, and your e-mail ends up spread out across all the different clients machines. Some POP clients try to partially account for this by having a "leave mail on server" option; but POP's inability to indicate read messages means downloading the same message multiple times, which is an inelegant solution at best. IMAP was designed with multiple clients in mind. Since the status of which e-mails have been read and which haven't (among other things) is stored on the server, you don't have to wade through seen messages even if you're connecting from a client machine you've never used before.
Conservation of bandwidth may not seem like a huge deal in this day of ubiquitous high-speed connections, until you're traveling and have your laptop on a slow dial-up connection and that 10MB attachment isn't nearly as critical as the information in the messages immediately after it. Because IMAP only transfers the actual messages you request, you don't have to wait for (or pay for) downloading spam, large attachments, or other e-mails you're not immediately interested in. You can even download some MIME parts of a message and not others.
Multiple mail folders allow for better organization of your saved mail. While POP accounts for only a single INBOX, IMAP allows for multiple mailbox folders to be manipulated directly from your mail client. You can create, delete, rename and transfer or copy messages between different mailbox folders. Depending on you server setup, you may even be able to have hierarchical mailbox folders that contain both messages and other mailbox subfolders within them.
IMAP supports shared folders. Not only can more than one person access the shared folders, but IMAP will manage concurrent access. This is particularly useful for role-based accounts, such as a [email protected]«mp/e.com or a help desk support mailbox, which may be accessed by multiple administrators.
Searching is built in to the protocol. Searches are performed on the server side to again reduce the amount of data that needs to be transferred. Matching result sets are then returned to the client for selection. This can be a huge win for large archival mailboxes, and shouldn't be underestimated.
While a POP client that is kept online for long periods of time can be configured to poll the server occasionally for new mail, IMAP avoids this problem altogether. A client connected to an IMAP server can be notified directly of any new mail that arrives.
Just in case the basic status indicators of read/unread, answered, important, and so on, aren't sufficient for your needs, IMAP allows user-defined Status flags. This means you can mark messages in ways that are uniquely meaningful to you and your needs.
IMAP can even support non-e-mail applications. For example, you may set up a documentation repository as an anonymous read-only IMAP folder. This could then be available to your entire company and accessed with any IMAP client.
When the client performs its initial handshake with the server, it negotiates which capabilities are supported by both machines, and which are not. This structure makes it easy to add optional features to IMAP gradually, without a major upheaval to the protocol. For example, during the initial handshake, clients and servers can negotiate support for start-TLS-style encrypted connections, something that was not in the original IMAP specification.
For those with pay-by-the-minute Internet connections, or other specialized circumstances that make POP's "offline" mode seem attractive, IMAP offers a "disconnected" mode. In this mode, all new mail is copied down to the client for local processing. Deletions and status changes are kept recorded by the client, and synced back to the server next time you connect. While this trades off connection time for bandwidth usage, it will still maintain many of the other advantages of using IMAP, such as a centralized mail store accessible by multiple clients, or custom status flags.
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