The protocol Internet Message Access (English Internet Message Access Protocol , or IMAP ), is an application protocol that allows access to messages stored on an Internet server. Using IMAP, you can access email from any computer that has an Internet connection. IMAP has several advantages over POP (another protocol used to get mail from a server). For example, it is possible to specify server-side folders in IMAP. On the other hand, it is more complex than POP since it allows you to view messages remotely and not downloading messages like POP does.
IMAP and POP3 (Post Office Protocol version 3) are the two most prevalent protocols for obtaining email. Virtually all email servers and clients are supported by both, although in some cases there are some typically proprietary manufacturer-specific interfaces. For example, the proprietary protocols used between the Microsoft Outlook client and your Microsoft Exchange Server or IBM’s Lotus Notes client and the Domino server. However, these products also support IMAP and POP3 interoperability with other clients and servers. The current version of IMAP, IMAP version 4 revision 1 (IMAP4rev1), is defined by RFC 3501.
IMAP was designed as a modern alternative to POP by Mark Crispin in 1986. Fundamentally, the two protocols allow mail clients to access messages stored on a mail server.
Whether using POP3 or IMAP4 to get messages, clients use SMTP to send messages. Email clients are commonly called POP or IMAP clients, but SMTP is used in both cases.
Summary
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- 1 Advantages over POP3
- 1 Connected and disconnected operating modes
- 2 Connection of multiple clients simultaneously to the same mailbox
- 3 Access to MIME parts of messages and partial fetch
- 4 Message status information
- 5 Multiple mailboxes on the server
- 6 Server-side searches
- 7 Extension mechanism
- 2 IMAP versions
- 1 IMAP2
- 2 IMAP3
- 3 IMAP2 bis
- 4 IMAP4
- 3 See also
- 4 References
- 5 External links
Advantages over POP3
The following table lists the main advantages of IMAP over POP3:
| Protocol | Advantage | Disadvantages |
| IMAP4 | · It works in permanent connection mode, so it notifies immediately of the arrival of new mail
· Transmits only the headers so the user can decide to delete them immediately · The download of the message occurs only when the user wants to read it · Local storage of the message is optional (a mail client option) · Manage folders, templates and drafts on the server · The storage of messages and folders on the server allows their use from multiple devices and simultaneously · It allows the search of messages by means of keywords · Messages can be tagged. The markup remains on the server · Shared folders can be created with other users (server dependent) |
· Not all email clients support the IMAP IDLE extension (notice of new emails)
· You need a transaction for each email you want to read · There is a delay in the message appearing on the user’s screen, while downloading · If the connection is lost, the message cannot be seen unless the mail client has stored it locally · Folders, templates and drafts cannot be read using POP (except the Inbox) |
| POP3 | · Emails appear immediately because they remain resident on the device (once downloaded) | · It only connects periodically every X minutes to check for new mail
· Periodic connection causes increased traffic and delayed client response (wait for full download) · In each connection, all new emails are downloaded, whether they will be read later or not · Emails take up local space on the device · By default, it deletes messages from the server, making it impossible to access them from another device |
Online and offline operating modes
When using POP, clients connect to the mail server briefly, only as long as it takes them to download new messages. Using IMAP4, clients stay connected as long as their interface is up and download messages on demand. This way of working for IMAP can give faster response times for users who have a large number of messages or large messages.
Connection of multiple clients simultaneously to the same mailbox
The POP protocol requires that the connected client be the only one connected to the mailbox. In contrast, the IMAP4 protocol allows simultaneous access to multiple clients and provides certain mechanisms for clients to detect changes made to a mailbox by another connected client. See, for example, RFC3501, section 5.2 which specifically says “simultaneous access to the same mailbox by multiple agents”.
Access to MIME parts of messages and partial fetch
Almost all Internet email is transmitted in MIME format, allowing messages to have a tree structure, where leaves are of a variety of content types in a single part, and non-leaf nodes are of a variety of multipart types. The IMAP4 protocol allows clients to get any individual MIME part separately and also to get portions of the individual parts or entire messages. These mechanisms allow clients to get the text portion of a message without having to download attachments or streams.
Message status information
Through the use of signals defined in the IMAP4 protocol, clients can follow the status of the message: for example, whether the message has been read or not, replied to or deleted. These signals are stored on the server, so that several clients connected to the same mailbox, at different times, can detect the changes made by other clients. POP does not offer any mechanism for clients to store this status information on the server, so if a user accesses a mailbox with two different POP clients (at different times), the status information – such as whether the message has been accessed – cannot sync between clients. The IMAP4 protocol supports both the predefined system of system signals, as well as that of defined keywords. System signals indicate status information, such as whether the message has been read. Keywords, which are not supported on all IMAP servers, allow messages to be given one or more labels whose meaning depends on the client. IMAP keywords should not be confused with proprietary tags of web-based mail services, which are translated into IMAP folders by some web-based servers by the corresponding proprietary servers.
Multiple mailboxes on the server
IMAP4 clients can create, rename, and / or delete mailboxes (usually presented as folders to the user) on the server, and copy messages between mailboxes. Support for multiple mailboxes also allows servers to provide access to shared and public folders. The IMAP4 Access Control List (ACL) Extension (RFC 4314) can be used to regulate access rights.
Server-side searches
IMAP4 provides a mechanism for clients to ask the server to search for messages according to a variety of criteria. This mechanism prevents clients from downloading all messages from their mailbox, thus speeding up searches.
Extension mechanism
Reflecting experience in previous Internet protocols, IMAP defines an explicit mechanism by which it can be extended. Many IMAP4 extensions have been proposed and are in common use. IMAP2bis does not have such a mechanism, and POP has one defined in RFC 2449.
An example of an extension is the IMAP IDLE , which is used for the server to notify the client when a new email message has arrived and they are synchronized. Without this extension, to perform the same task, the client would have to periodically contact the server to see if there are new messages.
IMAP versions
IMAP gives access to e-mail so that clients can keep local copies of messages (considered as a temporary cache ). In addition, it has multiple versions, among which are: IMAP2, IMAP3, IMAP2bis and IMAP4.
IMAP2
The IMAP2 version was the first publicly distributed version of IMAP and was defined in RFC 1064 in 1988. Later, it was updated by RFC 1176, it sought to deal with centralized email management that POP2 lacked. Labeling commands and responses were entered into it.
IMAP3
IMAP3 is a rare version of IMAP, it was specifically a counterpart of RFC 1176, which was defined by RFC 1203 in 1991 but it was not accepted by the market.
IMAP2 bis
The post office protocol leaves much of the management to a user who remains subject to a single machine or computer, if the equipment fails, all their data is lost and emails cannot be recovered, due to this and With the advent of MIME, the decision was made to extend IMAP2 to support MIME and to add functions for mailbox management. Then IMAP2 bis considered an experimental revision adds these functionalities of creating, deleting and saving messages as drafts in the respective mailboxes.
IMAP4
IMAP decided to make a change in the name of IMAP2 bis to avoid confusion with the proposal given in 1991 (IMAP3), which was made by a group of the competition that did not obtain achievements with said proposal, the new name given of IMAP2 bis would then be IMAP4. IMAP4 allows email clients to manipulate email messages stored on the server, as well as manipulate local folders. However, it was reviewed since they found or discovered security flaws. In 2003, IMAP4rev1 appeared, which included the functions of listening to messages before downloading the entire body of the email.
