Internet Message Access Protocol
Encyclopedia
Internet message access protocol (IMAP) is one of the two most prevalent Internet standard
protocols for e-mail
retrieval, the other being the Post Office Protocol
(POP). Virtually all modern e-mail client
s and mail servers support both protocols as a means of transferring e-mail messages from a server.
Internet protocol that allows an e-mail client
to access e-mail
on a remote mail server. The current version, IMAP version 4 revision 1 (IMAP4rev1), is defined by RFC 3501. An IMAP server listens on well-known port 143.
IMAP supports both on-line and off-line modes of operation. E-mail 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 e-mail clients support IMAP in addition to POP to retrieve messages; however, fewer email services support IMAP. IMAP offers access to the mail storage. Clients may store local copies of the messages, but these are considered to be a temporary cache.
Incoming e-mail messages are sent to an e-mail server that stores messages in the recipient's email box. The user retrieves the messages with an e-mail client that uses one of a number of e-mail retrieval protocols. Some clients and servers preferentially use vendor-specific, proprietary protocol
s, but most support the Internet standard protocols, SMTP for sending e-mail and POP and IMAP for retrieving e-mail, allowing interoperability with other servers and clients. For example, Microsoft
's Outlook
client uses a proprietary protocol to communicate with a Microsoft Exchange Server
server as does IBM
's Notes
client when communicating with a Domino
server, but all of these products also support POP, IMAP, and outgoing SMTP. Support for the Internet standard protocols allows many e-mail clients such as Pegasus Mail
or Mozilla Thunderbird
(see comparison of e-mail clients
) to access these servers, and allows the clients to be used with other servers (see list of mail servers).
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.
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.F
, 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).
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
format, allowing messages to have a tree structure
where the leaf nodes are any of a variety of single part content types and the non-leaf nodes are any of a variety of multipart types. The IMAP4 protocol allows clients to separately retrieve any of the individual MIME parts and also to retrieve portions of either individual parts or the entire message. These mechanisms allow clients to retrieve the text portion of a message without retrieving attached files or to stream
content as it is being fetched.
whose meaning is up to the client. Adding user created tags to messages is an operation supported by some web-based email
services, such as Gmail
.
workarounds such as Maildir
or database backends.
The IMAP specification has been criticised for being insufficiently strict and allowing behaviours that effectively negate its usefulness. For instance, the specification states that each message stored on the server has a "unique id" to allow the clients to identify the messages they have already seen between sessions. However, the specification also allows these UIDs to be invalidated with no restrictions, practically defeating their purpose.
Unless the mail storage and searching algorithms on the server are carefully implemented, a client can potentially consume large amounts of server resources when searching massive mailboxes.
IMAP4 clients need to maintain a TCP/IP connection to the IMAP server in order to be notified of the arrival of new mail. Notification of mail arrival is done through in-band signaling
, which contributes to the complexity of client-side IMAP protocol handling somewhat. A private proposal, push IMAP
, would extend IMAP to implement push e-mail
by sending the entire message instead of just a notification. However, push IMAP has not been generally accepted and current IETF work has addressed the problem in other ways (see the Lemonade Profile
for more information).
Unlike some proprietary protocols which combine sending and retrieval operations, sending a message and saving a copy in a server-side folder with a base-level IMAP client requires transmitting the message content twice, once to SMTP for delivery and a second time to IMAP to store in a sent mail folder. This is remedied by a set of extensions defined by the IETF LEMONADE Working Group for mobile devices: URLAUTH (RFC 4467) and CATENATE (RFC 4469) in IMAP and BURL (RFC 4468) in SMTP-SUBMISSION. POP servers don't support server-side folders so clients have no choice but to store sent items on the client. Many IMAP clients can be configured to store sent mail in a client-side folder, or to BCC oneself and then filter the incoming mail instead of saving a copy in a folder directly. In addition to the LEMONADE "trio", Courier Mail Server
offers a non-standard method of sending using IMAP by copying an outgoing message to a dedicated outbox folder.
Like POP, IMAP is an e-mail only protocol. As a result, items such as contacts, appointments or tasks cannot be managed or accessed using IMAP.
Internet standard
In computer network engineering, an Internet Standard is a normative specification of a technology or methodology applicable to the Internet. Internet Standards are created and published by the Internet Engineering Task Force .-Overview:...
protocols for e-mail
E-mail
Electronic mail, commonly known as email or e-mail, is a method of exchanging digital messages from an author to one or more recipients. Modern email operates across the Internet or other computer networks. Some early email systems required that the author and the recipient both be online at the...
retrieval, the other being the Post Office Protocol
Post Office Protocol
In computing, the Post Office Protocol is an application-layer Internet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection. POP and IMAP are the two most prevalent Internet standard protocols for e-mail retrieval. Virtually all modern...
(POP). Virtually all modern e-mail client
E-mail client
An email client, email reader, or more formally mail user agent , is a computer program used to manage a user's email.The term can refer to any system capable of accessing the user's email mailbox, regardless of it being a mail user agent, a relaying server, or a human typing on a terminal...
s and mail servers support both protocols as a means of transferring e-mail messages from a server.
E-mail protocols
The Internet Message Access Protocol (commonly known as IMAP) is an Application LayerApplication layer
The Internet protocol suite and the Open Systems Interconnection model of computer networking each specify a group of protocols and methods identified by the name application layer....
Internet protocol that allows an e-mail client
E-mail client
An email client, email reader, or more formally mail user agent , is a computer program used to manage a user's email.The term can refer to any system capable of accessing the user's email mailbox, regardless of it being a mail user agent, a relaying server, or a human typing on a terminal...
to access e-mail
E-mail
Electronic mail, commonly known as email or e-mail, is a method of exchanging digital messages from an author to one or more recipients. Modern email operates across the Internet or other computer networks. Some early email systems required that the author and the recipient both be online at the...
on a remote mail server. The current version, IMAP version 4 revision 1 (IMAP4rev1), is defined by RFC 3501. An IMAP server listens on well-known port 143.
IMAP supports both on-line and off-line modes of operation. E-mail 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 e-mail clients support IMAP in addition to POP to retrieve messages; however, fewer email services support IMAP. IMAP offers access to the mail storage. Clients may store local copies of the messages, but these are considered to be a temporary cache.
Incoming e-mail messages are sent to an e-mail server that stores messages in the recipient's email box. The user retrieves the messages with an e-mail client that uses one of a number of e-mail retrieval protocols. Some clients and servers preferentially use vendor-specific, proprietary protocol
Proprietary protocol
In telecommunications, a proprietary protocol is a communications protocol owned by a single organization or individual.-Enforcement:Proprietors may enforce restrictions through patents and by keeping the protocol specification a trade secret...
s, but most support the Internet standard protocols, SMTP for sending e-mail and POP and IMAP for retrieving e-mail, allowing interoperability with other servers and clients. For example, Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
's Outlook
Microsoft Outlook
Microsoft Outlook is a personal information manager from Microsoft, available both as a separate application as well as a part of the Microsoft Office suite...
client uses a proprietary protocol to communicate with a Microsoft Exchange Server
Microsoft Exchange Server
Microsoft Exchange Server is the server side of a client–server, collaborative application product developed by Microsoft. It is part of the Microsoft Servers line of server products and is used by enterprises using Microsoft infrastructure products...
server as does IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
's Notes
Lotus Notes
Lotus Notes is the client of a collaborative platform originally created by Lotus Development Corp. in 1989. In 1995 Lotus was acquired by IBM and became known as the Lotus Development division of IBM and is now part of the IBM Software Group...
client when communicating with a Domino
IBM Lotus Domino
Lotus Domino is an IBM server product that provides enterprise-grade e-mail, collaboration capabilities, and a custom application platform. Domino began life as Lotus Notes Server, the server component of Lotus Development Corporation's client-server messaging technology. It can be used as an...
server, but all of these products also support POP, IMAP, and outgoing SMTP. Support for the Internet standard protocols allows many e-mail clients such as Pegasus Mail
Pegasus Mail
Pegasus Mail is a donationware , proprietary, email client that is developed and maintained by David Harris and his team. It was originally released in 1990 for internal and external mail on Netware networks with MS-DOS clients, and was subsequently ported to Microsoft Windows...
or Mozilla Thunderbird
Mozilla Thunderbird
Mozilla Thunderbird is a free, open source, cross-platform e-mail and news client developed by the Mozilla Foundation. The project strategy is modeled after Mozilla Firefox, a project aimed at creating a web browser...
(see comparison of e-mail clients
Comparison of e-mail clients
The following tables compare general and technical features of a number of email client programs. Please see the individual products articles for further information. This article is not all-inclusive or necessarily up to date.-General:...
) to access these servers, and allows the clients to be used with other servers (see list of mail servers).
History
IMAP was designed by Mark CrispinMark Crispin
Mark Crispin is best known as the father of the IMAP protocol, having invented it in 1985 during his time at the Stanford Knowledge Systems Laboratory. He is the author or co-author of numerous RFCs; and is the principal author of UW IMAP, one of the reference implementations of the IMAP4rev1...
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 XeroxXerox
Xerox Corporation is an American multinational document management corporation that produced and sells a range of color and black-and-white printers, multifunction systems, photo copiers, digital production printing presses, and related consulting services and supplies...
Lisp machine
Lisp machine
Lisp machines were general-purpose computers designed to efficiently run Lisp as their main software language. In a sense, they were the first commercial single-user workstations...
client and a TOPS-20
TOPS-20
The TOPS-20 operating system by Digital Equipment Corporation was the second proprietary OS for the PDP-10 mainframe computer. TOPS-20 began in 1969 as the TENEX operating system of Bolt, Beranek and Newman...
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.F
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 command/response tagging and was the first publicly distributed version.IMAP3
IMAP3 is an extinct and 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 MIMEMIME
Multipurpose Internet Mail Extensions is an Internet standard that extends the format of email to support:* Text in character sets other than ASCII* Non-text attachments* Message bodies with multiple parts...
, 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
Pine (e-mail client)
Pine is a freeware, text-based email client developed at the University of Washington. The first version of this client was written in 1989. Source code was available for only the Unix version under a license written by the University of Washington...
were widely distributed with IMAP2bis support (Pine 4.00 and later supports IMAP4rev1).
IMAP4
An IMAP Working Group formed in the IETFInternet Engineering Task Force
The Internet Engineering Task Force develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standards bodies and dealing in particular with standards of the TCP/IP and Internet protocol suite...
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
Connected and disconnected modes of operation
When using POP, clients typically connect to the e-mail server briefly, only as long as it takes to download new messages. When using IMAP4, clients often stay connected as long as the user interface is active and download message content on demand. For users with many or large messages, this IMAP4 usage pattern can result in faster response times.Multiple clients simultaneously connected to the same mailbox
The POP protocol requires the currently connected client to be the only client connected to the mailbox. In contrast, the IMAP protocol specifically allows simultaneous access by multiple clients and provides mechanisms for clients to detect changes made to the mailbox by other, concurrently connected, clients.Access to MIME message parts and partial fetch
Usually all Internet e-mail is transmitted in MIMEMIME
Multipurpose Internet Mail Extensions is an Internet standard that extends the format of email to support:* Text in character sets other than ASCII* Non-text attachments* Message bodies with multiple parts...
format, allowing messages to have a tree structure
Tree structure
A tree structure is a way of representing the hierarchical nature of a structure in a graphical form. It is named a "tree structure" because the classic representation resembles a tree, even though the chart is generally upside down compared to an actual tree, with the "root" at the top and the...
where the leaf nodes are any of a variety of single part content types and the non-leaf nodes are any of a variety of multipart types. The IMAP4 protocol allows clients to separately retrieve any of the individual MIME parts and also to retrieve portions of either individual parts or the entire message. These mechanisms allow clients to retrieve the text portion of a message without retrieving attached files or to stream
Streaming media
Streaming media is multimedia that is constantly received by and presented to an end-user while being delivered by a streaming provider.The term "presented" is used in this article in a general sense that includes audio or video playback. The name refers to the delivery method of the medium rather...
content as it is being fetched.
Message state information
Through the use of flags defined in the IMAP4 protocol, clients can keep track of message state; for example, whether or not the message has been read, replied to, or deleted. These flags are stored on the server, so different clients accessing the same mailbox at different times can detect state changes made by other clients. POP provides no mechanism for clients to store such state information on the server so if a single user accesses a mailbox with two different POP clients, state information—such as whether a message has been accessed—cannot be synchronized between the clients. The IMAP4 protocol supports both pre-defined system flags and client defined keywords. System flags indicate state information such as whether a message has been read. Keywords, which are not supported by all IMAP servers, allow messages to be given one or more tagsTag (metadata)
In online computer systems terminology, a tag is a non-hierarchical keyword or term assigned to a piece of information . This kind of metadata helps describe an item and allows it to be found again by browsing or searching...
whose meaning is up to the client. Adding user created tags to messages is an operation supported by some web-based email
Web-based email
The term Webmail is used to describe two things. One use of the word is to describe a Webmail client: an email client implemented as a web application accessed via a web browser. This article focuses in this use of the term...
services, such as Gmail
Gmail
Gmail is a free, advertising-supported email service provided by Google. Users may access Gmail as secure webmail, as well via POP3 or IMAP protocols. Gmail was launched as an invitation-only beta release on April 1, 2004 and it became available to the general public on February 7, 2007, though...
.
Multiple mailboxes on the server
IMAP4 clients can create, rename, and/or delete mailboxes (usually presented to the user as folders) on the server, and move messages between mailboxes. Multiple mailbox support also allows servers to provide access to shared and public folders. The IMAP4 Access Control List (ACL) Extension (RFC 4314) may be used to regulate access rights.Server-side searches
IMAP4 provides a mechanism for a client to ask the server to search for messages meeting a variety of criteria. This mechanism avoids requiring clients to download every message in the mailbox in order to perform these searches.Built-in extension mechanism
Reflecting the experience of earlier Internet protocols, IMAP4 defines an explicit mechanism by which it may be extended. Many extensions to the base protocol have been proposed and are in common use. IMAP2bis did not have an extension mechanism, and POP now has one defined by RFC 2449.Disadvantages
While IMAP remedies many of the shortcomings of POP, this inherently introduces additional complexity. Much of this complexity (e.g., multiple clients accessing the same mailbox at the same time) is compensated for by server-sideServer-side
Server-side refers to operations that are performed by the server in a client–server relationship in computer networking.Typically, a server is a software program, such as a web server, that runs on a remote server, reachable from a user's local computer or workstation...
workarounds such as Maildir
Maildir
The Maildir e-mail format is a common way of storing e-mail messages, where each message is kept in a separate file with a unique name, and each folder is a directory...
or database backends.
The IMAP specification has been criticised for being insufficiently strict and allowing behaviours that effectively negate its usefulness. For instance, the specification states that each message stored on the server has a "unique id" to allow the clients to identify the messages they have already seen between sessions. However, the specification also allows these UIDs to be invalidated with no restrictions, practically defeating their purpose.
Unless the mail storage and searching algorithms on the server are carefully implemented, a client can potentially consume large amounts of server resources when searching massive mailboxes.
IMAP4 clients need to maintain a TCP/IP connection to the IMAP server in order to be notified of the arrival of new mail. Notification of mail arrival is done through in-band signaling
In-band signaling
In telecommunications, in-band signaling is the sending of metadata and control information in the same band or channel used for data.-Telephone:...
, which contributes to the complexity of client-side IMAP protocol handling somewhat. A private proposal, push IMAP
Push-IMAP
Push-IMAP, which is otherwise known as P-IMAP or Push extensions for Internet Message Access Protocol, is based on IMAPv4 Rev1 but contains additional enhancements for optimization in a mobile setting...
, would extend IMAP to implement push e-mail
Push e-mail
Push email is used to describe email systems that provide an always-on capability, in which new email is actively transferred as it arrives by the mail delivery agent to the mail user agent , also called the email client...
by sending the entire message instead of just a notification. However, push IMAP has not been generally accepted and current IETF work has addressed the problem in other ways (see the Lemonade Profile
Lemonade Profile
The Lemonade Profile is a set of protocols and mandatory extensions which provides email access to diverse environments, including mobile handsets and other resource constrained devices. It is the product of an IETF Working Group, and is largely based on pre-existing specifications, including IMAP...
for more information).
Unlike some proprietary protocols which combine sending and retrieval operations, sending a message and saving a copy in a server-side folder with a base-level IMAP client requires transmitting the message content twice, once to SMTP for delivery and a second time to IMAP to store in a sent mail folder. This is remedied by a set of extensions defined by the IETF LEMONADE Working Group for mobile devices: URLAUTH (RFC 4467) and CATENATE (RFC 4469) in IMAP and BURL (RFC 4468) in SMTP-SUBMISSION. POP servers don't support server-side folders so clients have no choice but to store sent items on the client. Many IMAP clients can be configured to store sent mail in a client-side folder, or to BCC oneself and then filter the incoming mail instead of saving a copy in a folder directly. In addition to the LEMONADE "trio", Courier Mail Server
Courier Mail Server
The Courier mail server is a mail transfer agent server that provides ESMTP, IMAP, POP3, SMAP, webmail, and mailing list services with individual components. It is best known for its IMAP server component....
offers a non-standard method of sending using IMAP by copying an outgoing message to a dedicated outbox folder.
Like POP, IMAP is an e-mail only protocol. As a result, items such as contacts, appointments or tasks cannot be managed or accessed using IMAP.
See also
- List of mail servers
- Comparison of e-mail clientsComparison of e-mail clientsThe following tables compare general and technical features of a number of email client programs. Please see the individual products articles for further information. This article is not all-inclusive or necessarily up to date.-General:...
- Post Office ProtocolPost Office ProtocolIn computing, the Post Office Protocol is an application-layer Internet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection. POP and IMAP are the two most prevalent Internet standard protocols for e-mail retrieval. Virtually all modern...
(POP) - Push-IMAPPush-IMAPPush-IMAP, which is otherwise known as P-IMAP or Push extensions for Internet Message Access Protocol, is based on IMAPv4 Rev1 but contains additional enhancements for optimization in a mobile setting...
- Simple Mail Access ProtocolSimple Mail Access ProtocolThe Simple Mail Access Protocol is an application layer Internet protocol for accessing e-mail stored on a server. It was introduced as part of the Courier suite, with the goal of creating a simpler and more capable alternative to IMAP....
- Webmail
- IMAP IDLEIMAP IDLEIn e-mail technology, IDLE is an IMAP feature described in RFC 2177 that allows a client to indicate to the server that it is ready to accept real-time notifications.- Significance :...
External links
- RFC 3501 - specification of IMAP version 4 revision 1
- RFC 2683 - IMAP Implementation Suggestions RFC
- RFC 2177 - IMAP4 IDLE command