What Is IMAP?

Date First Published: 24th March 2022

Topic: Computer Networking

Subtopic: Internet Protocols

Article Type: Computer Terms & Definitions

Difficulty Level: 5/10

Learn more about what IMAP is in this article.

Short for Internet Message Access Protocol, IMAP is an alternative protocol to POP3. It is used for receiving mail and not sending. IMAP stores the email messages on a server. As a result, this allows multiple devices to access the same POP3 account. When an email is deleted by the user, it will be deleted from all other devices and moving mail to different folders will also be synchronised across all devices. IMAP is an application-layer protocol that communicates over port 143 and port 993 over SSL/TLS.

IMAP is used more than POP3 because of its capability to store emails on a mail server. For example, if a device that uses POP3 got lost or stopped working, the user would lose all of their past emails. However, POP3 can save server storage space. When reading an IMAP message, it is not actually downloaded onto the device. It is read on the email service and the only time a message is downloaded is when they are clicked on. Attachments are not automatically downloaded, which makes reading messages quicker. Most email applications, such as Microsoft Outlook and Gmail allow users to specify the type of protocol that is used by the email server in the configuration settings.

Security

Similar to POP3 and SMTP, the lack of authentication and encryption capabilities in IMAP has made this protocol vulnerable to interception by attackers. This is because IMAP transfers sensitive information, such as usernames, passwords, and the contents of messages in plain text. However, there is a secure version of IMAP, similar to other protocols. It is called IMAPS, which stands for Internet Mail Access Protocol Secure, which encrypts sent data into ciphertext by the use of SSL/TLS so that it cannot be read by users if intercepted.

Versions

In 1986, Mark Crispin designed IMAP as a remote access mailbox protocol as an alternative to POP, the protocol used for retrieving an email from a mail server, downloading it to the device, and then deleting it from the server. A number of versions of IMAP exist, which are stated below.

Original IMAP

Originally called Interim Mail Access Protocol, this protocol was implemented as a Xerox Lisp Machine application and a TOPS-20 server. There are no copies of the original interim protocol specification or software. This original protocol lacked the command/response tagging and the syntax of this protocol was incompatible with other versions of IMAP.

IMAP 2

IMAP 2 replaced the original Interim Mail Access Protocol with Interactive Mail Access Protocol. It was defined in RFC 1064 in 1988 and updated later on by RFC 1176. The command/response tag was available in IMAP 2 and it was the first version that was distributed to the public.

IMAP 3

This is a very rare version of IMAP that was defined in RFC 1203 in 1991 and never approved by the marketplace. It was released as a proposal to RFC 1176 in 1990.

IMAP2bis

The release of MIME (Multipurpose Internet Mail Extensions) extended IMAP 2 to be compatible with MIME body structures and add functionality to manage mailboxes, such as creating, deleting, and renaming folders that were not included in IMAP 2.

IMAP 4

This is exactly the same as IMAP 4. However, an IMAP group that was formed in the IETF in the early 1990s took control over the design of IMAP2bis and renamed it to IMAP 4 in order to avoid confusion with IMAP 2.

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