The Internet Message Access Protocol (commonly known as IMAP or IMAP4, and previously called Internet Mail Access Protocol) is an application layer Internet protocol that allows a local client to access e-mail on a remote server. The current version, IMAP version 4 revision 1 (IMAP4rev1), is defined by RFC 3501. IMAP4 and POP3 (Post Office Protocol version 3) are the two most prevalent Internet standard protocols for e-mail retrieval. Virtually all modern e-mail clients and servers support both.

About E-mail Protocols

E-mail messages are generally sent to an e-mail server that stores received messages in the recipient's e-mail mailbox. The user later retrieves these messages with either a web browser or an e-mail client that uses one of a number of e-mail retrieval protocols. While some clients and servers preferentially use vendor specific, typically proprietary protocols, most support the Internet standard protocols SMTP for sending e-mail and POP3 and IMAP4 for retrieving e-mail, allowing interoperability with other servers and clients. For example, Microsoft's Outlook client typically uses a proprietary protocol to communicate with an Exchange server as does IBM's Notes client when communicating with a Domino server, but all of these products also support SMTP, POP3, and IMAP4. Support for the Internet standard protocols allows many other e-mail clients such as Qualcomm's Eudora or Mozilla's Thunderbird to access these servers and similarly allows the clients to be used with other servers.

E-mail clients can generally be configured to use either POP3 or IMAP4 to retrieve e-mail and in both cases use SMTP for sending. Most e-mail programs can also use Lightweight Directory Access Protocol (LDAP) for directory services.

Nearly all subscribers to individual Internet service provider e-mail accounts access their e-mail with client software that uses POP3.

IMAP is often used in large networks; for example, a college campus mail system. IMAP allows users to access new messages instantly on their computers, since the mail is stored on the network. With POP3, users either download the e-mail to their computer or access it via the web. Both methods take longer than IMAP, and the user must either download any new mail or "refresh" the page to see the new messages.

History

IMAP was designed by Mark Crispin in 1986 [1] as a remote Mailbox protocol, in contrast to the widely used POP, a protocol for retrieving the contents of a Mailbox.

IMAP 4

The current version of IMAP, IMAP version 4 revision 1 (IMAP4rev1), is defined by RFC 3501.

Unlike many older Internet protocols, IMAP4 natively supports encrypted login mechanisms. Plain-text transmission of passwords in IMAP4 is also possible. Because the encryption mechanism to be used must be agreed between the server and client, plain-text passwords are used in some combinations of clients and servers (typically Microsoft Windows clients and non-Windows servers). It is also possible to encrypt IMAP4 traffic using SSL, either by tunneling IMAP4 communications over SSL on port 993, or by issuing STARTTLS within an established IMAP4 session (see RFC 2595).

IMAP4 works over a TCP/IP connection using network port 143.

Advantages over POP3

• Both connected and disconnected modes of operation

When using POP3, clients typically connect to the e-mail server very briefly, only as long as it takes to download any 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 much faster response times.

• Multiple clients can simultaneously connect to the same mailbox

The POP3 protocol assumes the currently connected client is 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 parts of messages and partial fetch

Nearly all internet e-mail is transmitted in MIME format. MIME allows 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.

• Message state information kept on the server

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 multiple clients accessing the same mailbox at different times can detect state changes made by other clients. The protocol, but not all IMAP servers, supports the creation of client specified flags allowing messages to be given one or more "tags". This operation is supported by some Webmail services, such as Gmail, although generally not using IMAP.

• 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.

• 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.

• A built-in extension mechanism

Reflecting the experience of earlier Internet protocols, IMAP defines an explicit mechanism by which it may be extended. Many extensions to the base protocol have been proposed and are in common use. (To be fair, IMAP2bis did not have an extension mechanism, and POP3 now has one defined by RFC 2449.)

Disadvantages of IMAP

• IMAP is a very heavy and complicated protocol. Implementing IMAP is more difficult and error-prone than implementing POP3 for both client and server implementations. While true of early implementations, many modern imap servers and clients have overcome this and now use their default method.

• Server-side searches can potentially use lots of server resources when searching massive mailboxes.

• An IMAP4 client needs to explicitly request for new email messages causing additional delays on slow connections such as those commonly used by mobile devices. An extension to the protocol called "push IMAP" is currently under draft to implement "push e-mail" by sending the entire message instead of just a notification.

• Sending a message and saving a copy requires transmission of the message content twice, once to SMTP for delivery and a second time to IMAP to store in a sent mail folder. The Courier Mail Server used by Abiliba offers a method of sending by copying an outgoing message to a dedicated outbox folder but is not commonly supported in IMAP clients.