Internet Message Access Protocol (IMAP)

Overview

Internet Message Access Protocol, or IMAP, is a protocol used by email clients to retrieve email messages from a mail server. One could say that it is effectively in competition with POP because it accomplishes a function similar in nature, and most clients will force you to choose one or the other.. These days, the natural choice of protocol tends to be IMAP rather than POP.

The main difference between IMAP and POP is that messages are kept on the server, which keeps a kind of master reference. Instead of transferring the message to the client, only an image, or copy of the message is kept locally. When the client synchronises with the state of the server, it will tend to mimic that state. So if a new message is stored in a mailbox, it will appear on the client, or if a message is deleted from the server, it will also be deleted from the client.

The user can control the state of server from the client by adding, deleting, or moving messages between folders. The user can also add folders from the client, the state of which will be stored on the server.

Specifications

IMAP

Apparently, no official specification exists for the original IMAP. It became a specification with the publication of IMAP2

IMAP2

RFC1064

RFC1064 ("Interactive Mail Access Protocol - Version 2") was the original IMAP2 specification, published in 1988. The specification was obsoleted by RFC1176.

RFC1176

RFC1176 ("Interactive Mail Access Protocol - Version 2") provides various, somewhat minor updates to RFC1064. This specification was supseded by the first IMAP3 specification, RFC1203.

IMAP3

RFC1203

RFC1203 ("Interactive Mail Access Protocol - Version 3") obsoletes RFC1064, and presumedly also RFC1176. It appears to be the only version of IMAP3 that was published. Apparently, IMAP3 is a rare version, and was never really adopted by the marketplace.

IMAP4

RFC1730

RFC1730 ("Interactive Message Access Protocol - Version 4") was the first version of IMAP4, published in 1994. Note that the "M" in the name was changed from "Mail" to "Message". It was obsoleted two years later by RFC2060.

RFC2060

RFC2060 ("Interactive Message Access Protocol - Version 4rev1") was the first revision of the original IMAP4 specification. It was obsoleted by RFC3501.

RFC3501

RFC3501 ("Interactive Message Access Protocol - Version 4rev1") is the current IMAP4 standard in use. Though the name is confusing, is was the second revision since the publication of the original IMAP4 specification. This version was published in 2003. Likely, there has not been a need to update this specification as instead updates are published as extensions. Extensions to IMAP have been published as recently as in 2018.

Extensions to RFC3501

For brevity, we simply refence a number of extentions to RFC3501:

  • RFC4466 ("Collected Extensions to IMAP4 ABNF")

  • RFC4469 ("Internet Message Access Protocol (IMAP) CATENATE Extension")

  • RFC4551 ("IMAP Extension for Conditional STORE Operation or Quick Flag Changes Resynchronization")

  • RFC5032 ("WITHIN Search Extension to the IMAP Protocol")

  • RFC5182 ("IMAP Extension for Referencing the Last SEARCH Result")

  • RFC5738 ("IMAP Support for UTF-8")

  • RFC6186 ("Use of SRV Records for Locating Email Submission/Access Services")

  • RFC6858 ("Simplified POP and IMAP Downgrading for Internationalized Email")

  • RFC7817 ("Updated Transport Layer Security (TLS) Server Identity Check Procedure for Email-Related Protocols")

  • RFC8314 ("Cleartext Considered Obsolete: Use of Transport Layer Security (TLS) for Email Submission and Access")

  • RFC8437 ("IMAP UNAUTHENTICATE Extension for Connection Reuse")

  • RFC8474 ("IMAP Extension for Object Identifiers")

Related Concepts

IMAP is related to…​

  • POP as a "competing" protocol

  • Email clients, as the client will provide the functionality of an IMAP Client and connect to the IMAP Server

  • SMTP, in that messages will normally be delivered to the IMAP Server via SMTP.

Try It!

You can connect manually to James using the IMAP protocol by opening a openssl session.

Requirements:

  • Run the long demo (without stopping)

  • openssl s_client

Approximate duration: 5 minutes

Note: extra lines are inserted below to help show you what you need to type.

$ openssl s_client -connect 127.0.0.1:993
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
* OK JAMES IMAP4rev1 Server  Server faef5a990fbb is ready.

01 login user01@james.local 1234 (1)

01 OK LOGIN completed.

l list "" "*" (2)

* LIST (\HasNoChildren) "." "INBOX"
l OK LIST completed.

s select INBOX (3)

* FLAGS (\Answered \Deleted \Draft \Flagged \Seen)
* 1 EXISTS
* 1 RECENT
* OK [UIDVALIDITY 749313882] UIDs valid
* OK [UNSEEN 1] MailboxMessage 2 is first unseen
* OK [PERMANENTFLAGS (\Answered \Deleted \Draft \Flagged \Seen \*)] Limited
* OK [HIGHESTMODSEQ 5] Highest
* OK [UIDNEXT 2] Predicted next UID
s OK [READ-WRITE] SELECT completed.

f FETCH 1:* (FLAGS BODY.PEEK[HEADER.FIELDS (SUBJECT)]) (4)

* 1 FETCH (FLAGS (\Seen) BODY[HEADER.FIELDS (SUBJECT)] {75}
Subject: Test Message

)
f OK FETCH completed.

d store 1 +FLAGS (\Deleted) (5)

* 1 FETCH (FLAGS (\Deleted \Seen))
d OK STORE completed.

e expunge (6)

* 1 EXPUNGE
e OK EXPUNGE completed.

l logout (7)
1 Login as user user01@james.local
2 List the contents of the Mailbox
3 Select INBOX as the current folder
4 Fetch the contents of the INBOX folder, showing only the subject
5 Mark the message for deletion
6 Epunge the INBOX by deleting mails marked for deletion
7 Exit the session