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Internet Message Access Protocol
The Internet Message Access Protocol (also know as Internet Mail Access Protocol, Interactive Mail Access Protocol, and Interim Mail Access Protocol, shortly IMAP) is the most prevalent Internet standard protocol for e-mail retrieval. Virtually all modern e-mail clients and servers support both protocols as a means of transferring e-mail messages from a server, such as those used by Gmail, to a client, such as Mozilla Thunderbird, Apple Mail and Microsoft Outlook. Once configured, the client's use of such protocols remains transparent to the user.
IMAP is an application layer Internet protocol operating on port 143 that allows a local client to access e-mail on a remote server. The current version is the IMAP version 4 revision 1.
IMAP supports both connected and disconnected modes of operation. E-mail clients using this protocol generally leave messages on the server until the user explicitly deletes them. This and other facets of IMAP operation allow multiple clients to access the same mailbox. Most e-mail clients support either POP3 or IMAP to retrieve messages, but fewer Internet Service Providers (ISPs) support IMAP.
IMAP is often used in large networks, for example, a college campus mail system. IMAP allows users to access new messages as fast as the network can deliver them to their computers. With POP3, users either download the e-mail to their computer or access it via the web. Both methods take longer than IMAP over a local network, and the user must download any new mail to see the new messages.
Advantages over POP3
Connected and disconnected modes of operation
When using POP3, 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 POP3 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.
The POP3 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.
Message state information
Through the use of flags defined in the IMAP4 protocol, clients can keep track of message state. 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. POP3 provides no mechanism for clients to store such state information on the server so if a single user accesses a mailbox with two different POP3 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.
Through the use of flags defined in the IMAP4 protocol, clients can keep track of message state. 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. POP3 provides no mechanism for clients to store such state information on the server so if a single user accesses a mailbox with two different POP3 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.
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.
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.
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.
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.
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.
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