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IMAP

IMAP
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The essential point is that with the online paradigm, one's incoming and archive message folders are stored on a server and may be accessed uniformly from different computers at different times, without relying on general purpose file system protocols (which are not uniformly available on all platforms, and which may also introduce performance and file locking problems). This is not an important goal for those who always use the same computer to access their email, but it is a very important one for those who use multiple computers.

With that background, here is a brief comparison of POP and IMAP technologies:

* Characteristics common to both POP and IMAP:
-Both can support offline operation.
-Mail is delivered to a shared, "always up" mail server.
-New mail accessible from a variety of client platform types.
-New mail accessible from anywhere in network.
-Protocols are open; defined by Internet RFCs.
-Freely available implementations (including source) available.
-Clients available for PCs, Macs, and Unix.
-Commercial implementations available.
-Internet oriented; no SMTP mail gateways required.
-Protocols deal with access only; both rely on SMTP to send.
-Both support persistent message IDs (for disconnected operation).

* POP protocol advantages:
-Simpler protocol; easier to implement.
-More client software currently available.

* IMAP protocol advantages:
-Can manipulate persistent message status flags.
-Can store messages as well as fetch them.
-Can access and manage multiple mailboxes.
-Can support concurrent updates and access to shared mailboxes.
-Suitable for accessing non-email data; e.g., NetNews, documents.
-Can also use offline paradigm, for minimum connect time and disk use.
-Companion protocol defined for user configuration management (IMSP).
-Constructs to permit online performance optimization, especially over low-speed links.

Elaborating on these points:

IMAP can manipulate persistent message status flags. These include flags such as "Seen", "Deleted", "Answered", as well as user-defined flags.

IMAP can store messages as well as fetch them. One can append a message from an incoming message folder to an archive folder (or vice versa).

IMAP can access and manage multiple mailboxes. This includes the ability to name and access different incoming and archive message folders, but also the ability to list, create, delete, and rename them. These mailboxes can be on the same server or on different servers. An IMAP client may allow you to see them at the same time, and move messages from one to the other.

IMAP can support concurrent updates and access to shared mailboxes. This capability is useful when multiple individuals are processing messages coming into a common inbox. Changes in mailbox state can be presented to all concurrently active clients via IMAP.

IMAP is suitable for accessing non-email data; e.g., NetNews, documents. This is handy for uniformly accessing different classes of information.

IMAP can also support the offline paradigm, for minimum connect time and server resources. The offline paradigm is useful in situations where the only access to a mail server is via expensive dialup connections and multi-platform access to one's mailboxes is not needed. It is also useful in environments where client machines are resource-rich and servers are resource-poor. Not all IMAP clients offer good offline processing support, but the protocol is certainly capable of it.

IMAP has a companion protocol defined for user configuration management called IMSP, the Internet Message Support Protocol. IMSP permits location-independent (multi-platform) access to personal configuration data such as address books.

IMAP has constructs to permit online performance optimization, especially over low-speed links. These include the ability to fetch the structure of a message without downloading it, to selectively fetch individual message parts, and the ability to use the server for searching in order to minimize data transfer between client and server.

Especially when connecting to a mail server via low-bandwidth lines, it is useful to be able to defer transferring messages or parts of messages that are not of immediate interest until a more propitious time. With multimedia or multipart MIME messages, transferring selected parts of a message can be a huge advantage, as when one is in a hotel room and has just received a short text message with a 10MB video clip attached. Efficient processing of MIME messages is a significant advantage of IMAP over POP. (MIME stands for Multipurpose Internet Mail Extensions. It is the Internet standard method for sending arbitrary files as attachments to SMTP and RFC-822 compatible Internet mail messages.)

In summary, IMAP offers advantages over POP in three areas: richer functionality in manipulating one's inbox, the ability to manage mail folders besides one's inbox, and primitives to allow optimization of online performance, especially when dealing with large MIME messages.

Because there are freely available IMAP development libraries, its additional complexity over POP should not be a significant barrier to use. Therefore, a reasonable conclusion is that the only advantage of POP over IMAP is that there is currently more POP software available. However, this is changing rapidly, and IMAP's functional advantages over POP are nothing less than overwhelming.

(taken from text at imap.org)