What Is IMAP

Email Tutorials - Herong's Tutorial Examples

∟What Is IMAP

This section provides a quick introduction on IMAP (Internet Message Access Protocol) for retrieving email messages. An IMAP sample session of retrieving an email message using the 'telnet' command is included.

What Is IMAP? - IMAP (Internet Message Access Protocol) is a communication protocol used by MUA (Message User Agent) to retrieve email messages stored by a MDA (Message Delivery Agent). See the diagram "Email Processing and Software Agents / Protocols" listed in the last section.

The original version of IMAP was published in 1988 by M. Crispin in "RFC1064 - Internet Message Access Protocol - Version 2" ( datatracker.ietf.org/doc/html/rfc1064).

The latest version of IMAP was published in 2021 by A. Melnikov and B. Leiba in "RFC1064 - Internet Message Access Protocol (IMAP) - Version 4rev2" ( datatracker.ietf.org/doc/html/rfc9051).

Here is a sample session of retrieving an email message with IMAP using "telnet" as the MUA. A Dovecot server is used to access email messages stored by a MDA.

herong$ telnet 127.0.0.1 143
  Trying 127.0.0.1...
  Connected to 127.0.0.1.
  Escape character is '^]'.
  * OK [CAPABILITY IMAP4rev1 SASL-IR LOGIN-REFERRALS ID ENABLE IDLE
    LITERAL+ STARTTLS AUTH=PLAIN AUTH=LOGIN] Dovecot ready.

c1 login herong TopSecret
  c1 OK [CAPABILITY IMAP4rev1 SASL-IR LOGIN-REFERRALS ID ENABLE IDLE
  SORT SORT=DISPLAY THREAD=REFERENCES THREAD=REFS THREAD=ORDEREDSUBJECT
  MULTIAPPEND URL-PARTIAL CATENATE UNSELECT CHILDREN NAMESPACE UIDPLUS
  LIST-EXTENDED I18NLEVEL=1 CONDSTORE QRESYNC ESEARCH ESORT SEARCHRES
  WITHIN CONTEXT=SEARCH LIST-STATUS BINARY MOVE SNIPPET=FUZZY
  PREVIEW=FUZZY LITERAL+ NOTIFY SPECIAL-USE] Logged in

c2 select "INBOX"
  * FLAGS (\Answered \Flagged \Deleted \Seen \Draft)
  * OK [PERMANENTFLAGS (\Answered \Flagged \Deleted \Seen \Draft \*)]
    Flags permitted.
  * 2 EXISTS
  * 0 RECENT
  * OK [UIDVALIDITY 1605272831] UIDs valid
  * OK [UIDNEXT 22] Predicted next UID
  c2 OK [READ-WRITE] Select completed (0.001 + 0.000 secs).

c3 fetch 1 RFC822
  * 1 FETCH (RFC822 {416}
  Return-Path: <herong@herongyang.com>
  X-Original-To: herong
  Delivered-To: herong@herongyang.com
  Received: from localhost (localhost [IPv6:::1])
    by mail.herongyang.com (Postfix) with ESMTP id 5EABD266002B
    for <herong>; 15 Nov 2023 06:19:34 -0500 (EST)
  Message-Id: <20231113111946.5EABD266002B@mail.herongyang.com>
  Date: 15 Nov 2023 06:19:34 -0500 (EST)
  From: herong@herongyang.com

  Hello world!
  )
  c3 OK Fetch completed (0.022 + 0.000 + 0.021 secs).

Note that retrieving email messages with IMAP is not secure, because data is transmitted in clear text. So most of email systems are not using IMAP directly anymore. They are using the IMAPS (Internet Message Access Protocol Secure) method to secure the IMAP communication. This is done by wrapping the IMAP communication under a SSL (Secure Socket Layer) or TLS (Transport Layer Security) connection. See next section for more details on IMAPS.