IMAP (Internet Message Access Protocol) is an Internet protocol that allows us to retrieve mail from a mail server over TCP / IP networks. The protocol was developed by Mark Crispin in 1986 and is RFC 3501 Internet standard no. 

IMAP is designed to provide the ability to fully manage email accounts using multiple email clients at the same time. Mail remains on the server until the user deletes it from one of the clients.

By default, the IMAP server communicates (unencrypted) on the 143 port, while the encrypted (IMAP) SSL or IMAPS) is on port 993.

Today’s modern email clients and servers generally all support IMAP. IMAP and the former POP3 (Post Office Protocol) are the most common standards for handling email, so they are also supported by most webmail providers, such as Gmail or even home webmail providers.

The protocol also supports on-line and off-line modes. Thanks to its functionalities, it is possible to manage the same email account from several clients simultaneously. Clients can store copies of mail on the server locally, but these copies only serve as caches.

Incoming mail is sent to the email server, which is stored in the recipient's email account. The user can access their mail account using an email client that uses one of the protocols supported by the mail server. Some servers and clients prefer the manufacturer's own protocols, but they also support the IMAP / POP3 standards for receiving mail and the SMTP protocol for sending mail, while maintaining compatibility between different servers and clients. For example, the Microsoft Outlook client uses MAPI, Microsoft's own protocol, to communicate with the Microsoft Exchange server, but it also supports POP3 / IMAP / SMTP protocols to connect to other mail servers. Support for these standard protocols allows many clients, such as Mozilla Thunderbird, to access most mail servers.

history

IMAP was designed as a remote mailbox protocol by Mark Crispin in 1986, unlike the then widely used POP, which was used to query the contents of a mailbox.

IMAP before Internet Mail Access Protocol, Interactive Mail Access Protocol and Interim Mail Access Protocol was known by names.

Original IMAP

The original Interim Mail Access Protocol was implemented as a Xerox Lisp machine client and TOPS-20 server. There was no copy of the protocol specification or software at that time, it existed solely in the mind of its designer.

Although some commands and responses were similar to IMAP2, this protocol had no command / response tagging, so its syntax was not compatible with later versions of IMAP.

IMAP2

The Interim Mail Access Protocol was soon replaced by the Interactive Mail Access Protocol (IMAP2), which was RFC 1064defined in (1988) and RFC 1176(1990). Command / response tagging was introduced in IMAP2 and was the first publicly distributed version.

IMAP3

IMAP3 is a very unique version of IMAP that it does RFC 1203was published in 1991. It was specifically written as a contradiction of RFC 1176, which itself suggested modifying IMAP2. IMAP3 has never been accepted by the market.

IMAP2bis

MIME (Multipurpose Internet Mail Extensions), IMAP2 was expanded with support for MIME structures and mailbox management features (writing, deleting, renaming, and uploading mail) that were missing from IMAP2. This experimental change was called IMAP2bis; the specification never appeared in non-sketchy form.

IMAP4

The responsibility for designing IMAP2bis lies with IETF(Internet Engineering Task Force) was taken over by the IMAP working group set up in the 1990s. The working group decided to rename IMAP2bis to IMAP4 to avoid confusion.

Its advantages over POP

• Connected and disconnected operating modes
  When using POP, clients usually connect to the mail server briefly, only to retrieve new messages. When using IMAP4, clients remain connected until the user interface is active and the message content is downloaded as needed. For users with large or large mail volumes, using IMAP4 results in faster response times.
• Multiple clients can join the same account at the same time
  Unlike POP, IMAP allows multiple users to connect to the same mailbox at the same time. Changes made to the mailbox are also synchronized to the devices of parallel clients.
• Full or partial download option
  The IMAP4 protocol allows users to download individual message sections individually. For example, users can download only the text of the message without having to download large attachments.
• Message status information
  Using the flags defined in the IMAP4 protocol, users can track the status of a message, such as a read / reply / delete message. These alerts are stored on the server so that other users who connect to the same account at the same time can notice these changes.
• Manage multiple mailboxes
  The protocol allows users to create accounts, or rename or delete accounts. These accounts are usually viewed by the user as folders, which are arranged in a tree structure. You can also create shared or public folders.
• Server side searches
  The client can perform searches on the server without having to download all mail.

Some of the more well-known IMAP servers

• Courier
• Cyrus IMAP server
• Dovecot
• FirstClass Server
• IceWarp Mail Server
• Mac OS X Server
• Microsoft Exchange Server
• Mirapoint
• Stalker Communigate Pro