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Internet Message Access Protocol is one of the two most common e-mail retrieval protocols. Also known by the acronym IMAP, it is an Internet protocol which operates at the Application Layer. With IMAP, a mailbox can be read and managed simultaneously by a number of different e-mail clients. IMAP is frequently used by a large percentage of Internet users to download e-mail from web mail servers.
Originally called Interim Mail Access Protocol, the first version of IMAP has undergone several revisions since it was created in 1986. Version 2 was published in 1988 as Request For Comments (RFC) 1064 by the Internet Engineering Task Force (IETF). It changed the IMAP acronym to Interactive Mail Access Protocol and it was revised again in 1990 by RFC 1176. A Version 2 enhancement supporting Multipurpose Internet Mail Extensions (MIME) is the basis for Version 4. Although it was proposed in 1993, IMAP Version 3 was never adopted and was discarded in favor of Version 4.
Now called Internet Message Access Protocol, Version 4 became the standard in 1994, published in RFC 1730. Design flaws caused it to be replaced in 1996 by IMAP Version 4 Revision 1, which was refined again in 2003 by RFC 3501. All prior versions and revisions are effectively obsolete and unused.
Post Office Protocol (POP) is the other common Internet protocol for e-mail retrieval. Most e-mail servers and clients support Internet Message Access Protocol and POP in addition to their own unique protocols. Compared to POP, IMAP has many advantages including the ability to load a portion of an e-mail rather than wait for all attachments. It can also stream message content using the MIME mechanism. IMAP clients also tend to stay connected to a mail server for longer periods, which can improve overall response time.
Changes to mailbox and message status made by other simultaneously-connected clients are stored on the mail server by IMAP. Since POP does not do this, it has no way to determine whether another client has already read a message, for example. Some Internet Message Access Protocol servers also support customized keyword tags attached to e-mail messages. IMAP also provides an efficient server-based system to search a mailbox for specific content without downloading all the messages.
Server mailbox and message management functions are included in IMAP, allowing clients to easily modify mailboxes. Extensions to the core Internet Message Access Protocol features are also supported. One of the more common extensions creates an efficient means for mobile devices to store copies of sent e-mails. IMAP also encrypts user and password information during login, rather than exposing it in plain text like some other protocols.