IMAP (Internet Messaging Access Protocol)

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Wi-Fi or cable, IMAP helps transport email over networks. ©jerry john; CC BY-ND 2.0 license


IMAP is an internet standard that describes a protocol for retrieving mail from an email (IMAP) server.

What Can IMAP Do?

Typically, messages are stored and organized in folders on the server. Email clients on computers and mobile devices replicate that structure, at least in part, and synchronize actions (such as deletion or moving messages) with the server.

That means multiple programs can access the same account and all show the same state and messages, all synchronized.

It allows you to move messages between email accounts seamlessly, have third-party services connect to your account to add functionality (for example, to automatically sort or back up messages).

IMAP is an acronym for Internet Messaging Access Protocol, and the protocol’s current version is IMAP 4 (IMAP4rev1).

How Does IMAP Compare to POP?

IMAP is a more recent and more advanced standard for mail storage and retrieval than POP (Post Office Protocol). It allows for messages to be kept in multiple folders, supports folder sharing, and online mail handling, say through a web browser, where email message need not be stored on the user's computer.

Is IMAP Also for Sending Mail?

The IMAP standard defines commands to access and operate on emails on a server. It does not include operations for sending messages. For sending email (both using POP and using IMAP for retrieval), SMTP (Simple Mail Transfer Protocol) is used.

Does IMAP Have Disadvantages?

As it is with sending mail, IMAP's advanced functions also come with complexities and ambiguities.

After a message has been sent (through SMTP), for example, it needs to be transmitted again (through IMAP) to be stored in the IMAP account's "Sent" folder.

IMAP is difficult to implement, and both IMAP email clients and servers may differ in how they interpret the standard.

Partial implementations and private extensions as well as inevitable bugs and quirks can make IMAP hard on the programmers and slow as well as less reliable than desired for users.

Email programs may start downloading full folders anew for no apparent reason, for example, and search can strain servers and make email slow for multiple users.

Where Is IMAP Defined?

The principal document to define IMAP is RFC (Request for Comments) 3501 from 2003.

Are There Any Extensions to IMAP?

The basic IMAP standard allows for extensions not only to the protocol but also to individual commands in it, and many have been defined or implemented.

Popular IMAP extensions include IMAP IDLE (real-time notifications of received email), SORT (sorting messages at the server so that the email program can fetch only the latest or biggest, for example, without having to download all emails) and THREAD (which lets email clients retrieve related messages without downloading all mail in a folder), CHILDREN (implementing a hierarchy of folders), ACL (Access Control List, specifying rights for individual users per IMAP folder)

A more complete list of IMAP extensions can be found at the Internet Message Access Protocol (IMAP) Capabilities Registry.

Gmail includes a few specific extension to IMAP, too.

(Updated December 2015)

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