What is Internet Message Access Protocol (IMAP)

Internet Message Access Protocol (IMAP) is an Internet standard protocol used by email clients to retrieve email messages from a mail server over a TCP/IP connection. IMAP is defined by RFC 9051.

IMAP was designed to permit complete management of an email box by multiple email clients, therefore clients generally leave messages on the server until the user explicitly deletes them. An IMAP server typically listens on port number 143. IMAP over SSL/TLS (IMAPS) is assigned port number 993.

Virtually all modern e-mail clients and servers support IMAP, which along with the earlier POP3 (Post Office Protocol) are the two most prevalent standard protocols for email retrieval. Many webmail service providers such as Gmail and Outlook.com also provide support for both IMAP and POP3.

Email protocols

The Internet Message Access Protocol is an application-layer Internet protocol that allows an e-mail client to access email on a remote mail server. The current version is defined by RFC 9051. An IMAP server typically listens on well-known port 143, while IMAP over SSL/TLS (IMAPS) uses 993.

Incoming email messages are sent to an email server that stores messages in the recipient’s email box. The user retrieves the messages with an email client that uses one of several email retrieval protocols. While some clients and servers preferentially use vendor-specific, proprietary protocols, almost all support POP and IMAP for retrieving email – allowing many free choices between many e-mail clients such as Pegasus Mail or Mozilla Thunderbird to access these servers and allows the clients to be used with other servers.

Email clients using IMAP generally leave messages on the server until the user explicitly deletes them. This and other characteristics of IMAP operation allow multiple clients to manage the same mailbox. Most email clients support IMAP in addition to Post Office Protocol (POP) to retrieve messages. IMAP offers access to mail storage. Clients may store local copies of the messages, but these are considered to be temporary cache.

 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 RFC 1176 (IMAP2) rather than RFC 1203 (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 from 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, RFC 1176, and RFC 1064 (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.