Internet Message Access Protocol

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Internet Message Access Protocol - IMAP is one of two Internet protocol more used to extract e-mails from server. The other protocol is POP (Post Office Protocol). All e-mail servers and e-mail clients supports both protocols.

E-mail protocols

IMAP protocol allows a e-mail client to remotely access the e-mail server. IMAP can work both online and offline. E-mail clients using IMAP leaves messages on the server until the user deletes them. This and other characteristics allow multiple users to manage the same e-mail address.

History

IMAP was designed by Mark Crispin in 1986 as remote mail protocol.

Original IMAP

Interim Access Protocol) was implemented as a Xerox Lisp machine client and as a TOPS-20 server.There is no copy of the original Interim protocol.

IMAP2

Interim protocol was quickly replaced by Interactive Mail Access Protocol (IMAP2). IMAP2 inserted command tag and was the first version distributed public.

IMAP2bis

Once with MIME(Multipurpose Internet Mail Extensions), IMAP2 was extended to support MIME structures and has been added new management features to the e-mail client(create, delete, rename, load messages) missing in IMAP2. This review was called IMAP2bis.

IMAP4

A IMAP work group formed IETF (Internet Engineering Task Force) in the early 1990 and took over the responsibilities of IMAP2bis project. They renamed IMAP2bis in IMAP4 to avoid confusions with IMAP3 competitor. The IMAP acronym changes into Internet Message Access Protocol.

Some designing faults in the original IMAP4 led to replace it with IMAP4rev1 two years later.

IMAP4rev1

IMAP4rev1 is compatible with IMAP2, IMAP2bis and IMAP4. Although older version are almost extinct.

Unlike other older Internet protocols, IMAP naturally supports encrypted authentication mechanisms. While IMAP servers can be configured to allow passwords transmission RFC 3501 empowers the support for authentication methods avoiding this vulnerability. It is possible the IMAP traffic encryption using Transport Layer Security (SSL) or by directing IMAP communications over SSL, port 993 or by issuing a command STARTTLS with an established IMAP session.

Advantages over POP

Modes of connection and disconnection

Using POP clients usually connects for a short time to the e-mail server enough to download new messages. When clients use IMAP4 they are connected to server as long as the user interface is active and download messages at requests. For users with many and large messages IMAP4 may bring faster answering times.

Multiple clients connected to same e-mail

POP protocol allows only one client connected to an e-mail account while IMAP allows multiple simultaneous access to an e-mail account.

Access to parts of MIME messages

Usually all e-mail messages are sent in MIME format allowing massages to have an arborescent structure where leaf nodes are any of the varieties of a single content type and non-leaf nodes are any of several varieties of content types. IMAP4 allows clients to take any distinct individual MIME part and they also can take portions from each individual part or the entire message.

Information regarding messages' state

To use different flags in IMAP protocol clients can allow messages' state; for example if a message has or has not been read if it was replied or deleted. These flags are stored on the server so different clients accessing the same e-mail account in different periods of time to be able to detect state changes realized by other clients. POP protocol does not offer any mechanism for clients to store such information on server.

Multiple e-mail accounts on the server

IMAP4 clients can create, rename and/or delete e-mail accounts from the server and can move messages between accounts. Support for multiple e-mail accounts also allows server to access shared and public folders.

Search on server

IMAP4 offer a mechanism for a client to request server to search messages after a specific criteria.

IMAP disadvantages

Except the case when messages storage and search algorithms from a server are well implemented, a e-mail client may consume server's resources when searches are performed in large e-mail accounts.

IMAP4 clients requests permanent TCP / IP connection with the IMAP server to be noticed about new messages.

Unlike some owned protocols combining sending operations with the recovery ones, sending a message and copying it in a folder on a IMAP server requires the message to be sent two times: first to SMTP for sending and second to IMAP for storage.

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