What is IMAP?

The following was culled from the UW IMAP site

What is IMAP?

IMAP stands for Internet Message Access Protocol. It is a method of accessing electronic mail or bulletin board messages that are kept on a (possibly shared) mail server. In other words, it permits a "client" email program to access remote message stores as if they were local. For example, email stored on an IMAP server can be manipulated from a desktop computer at home, a workstation at the office, and a notebook computer while traveling, without the need to transfer messages or files back and forth between these computers.

IMAP's ability to access messages (both new and saved) from more than one computer has become extremely important as reliance on electronic messaging and use of multiple computers increase, but this functionality cannot be taken for granted: the widely used Post Office Protocol (POP) works best when one has only a single computer, since it was designed to support "offline" message access, wherein messages are downloaded and then deleted from the mail server. This mode of access is not compatible with access from multiple computers since it tends to sprinkle messages across all of the computers used for mail access. Thus, unless all of those machines share a common file system, the offline mode of access that POP was designed to support effectively ties the user to one computer for message storage and manipulation.

Key goals for IMAP include:

  • Be fully compatible with Internet messaging standards, e.g. MIME.
  • Allow message access and management from more than one computer.
  • Allow access without reliance on less efficient file access protocols.
  • Provide support for "online", "offline", and "disconnected" access modes *
  • Support for concurrent access to shared mailboxes
  • Client software needs no knowledge about the server's file store format.

The protocol includes operations for creating, deleting, and renaming mailboxes; checking for new messages; permanently removing messages; setting and clearing flags; server-based RFC-822 and MIME parsing (so clients don't need to), and searching; and selective fetching of message attributes, texts, and portions thereof for efficiency.

IMAP was originally developed in 1986 at Stanford University. However, it did not command the attention of mainstream email vendors until a decade later, and it is still not as well-known as earlier and less-capable alternatives such as POP, though that is rapidly changing, as articles in the trade press and the implementation of IMAP in more and more software products show. (See IMAP Status and History for a chronological overview of significant IMAP developments.)

There is a companion protocol to IMAP, developed at Carnegie Mellon University. It is called the "Application Configuration Access Protocol", or ACAP, and provides the same location independent access to configuration files, address books, bookmark lists, etc, that IMAP offers for mailboxes.

* See Comparing Two Approaches to Remote Mailbox Access: IMAP vs. POP and Message Access Paradigms and Protocols for a detailed discussion of the advantages of IMAP as a message access method.

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