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Internet Message Access Protocol (IMAP) is
a protocol for e-mail retrieval and storage developed by
Mark Crispin in 1986 at Stanford University as an
alternative to POP. IMAP unlike POP, specifically allows
multiple clients simultaneously connected to the same
mailbox, and through flags stored on the server, different
clients accessing the same mailbox at the same or different
times can detect state changes made by other clients.
IMAP was designed by Mark Crispin in 1986 as a remote
mailbox protocol, in contrast to the widely used POP, a
protocol for retrieving the contents of a mailbox.
IMAP was previously known as Internet Mail Access Protocol,
Interactive Mail Access Protocol (RFC 1064), and Interim
Mail Access Protocol.
Original IMAP: The original Interim Mail Access Protocol was
implemented as a Xerox Lisp machine client and a TOPS-20
server. No copies of the original interim protocol
specification or its software exist. Although some of its
commands and responses were similar to IMAP2, the interim
protocol lacked command/response tagging and thus its syntax
was incompatible with all other versions of IMAP.
IMAP2: The interim protocol was quickly replaced by the
Interactive Mail Access Protocol (IMAP2), defined in RFC
1064 (in 1988) and later updated by RFC 1176 (in 1990).
IMAP2 introduced the command/response tagging and was the
first publicly distributed version.
IMAP3: IMAP3 is an extremely rare variant of IMAP. It was
published as RFC 1203 in 1991. It was written specifically
as a counter proposal to RFC 1176, which itself proposed
modifications to IMAP2. IMAP3 was never accepted by the
marketplace. The IESG reclassified RFC1203 "Interactive Mail
Access Protocol - Version 3" as a Historic protocol in 1993.
The IMAP Working Group used RFC1176 (IMAP2) rather than
RFC1203 (IMAP3) as its starting point.
IMAP2bis: With the advent of MIME, IMAP2 was extended to
support MIME body structures and add mailbox management
functionality (create, delete, rename, message upload) that
was absent in IMAP2. This experimental revision was called
IMAP2bis; its specification was never published in non-draft
form. An internet draft of IMAP2bis was published by the
IETF IMAP Working Group in October 1993. This draft was
based upon the following earlier specifications: unpublished
IMAP2bis.TXT document, RFC1176, and RFC1064 (IMAP2). The
IMAP2bis.TXT draft documented the state of extensions to
IMAP2 as of December 1992. Early versions of Pine were
widely distributed with IMAP2bis support (Pine 4.00 and
later supports IMAP4rev1).
IMAP4: An IMAP Working Group formed in the IETF in the early
1990s took over responsibility for the IMAP2bis design. The
IMAP WG decided to rename IMAP2bis to IMAP4 to avoid
confusion with a competing IMAP3 proposal from another group
that never got off the ground. The expansion of the IMAP
acronym also changed to the Internet Message Access
Protocol. The latest version, IMAP4, allows an email client
to manipulate email messages stored on a server in the same
way as a client using local folders. This ability allows
multiple clients for a single user to see the same mailbox
status. For example, if a user moves a message from the
user's INBOX to some other folder using one client, when
later accessing the mailbox from another client the message
appears in the folder to which it was moved.
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