Internet Message Access Protocol (IMAP)

IMAP (Internet Message Access Protocol) is a kind of a network file system for managing e-mail messages and attachments on a remote server.

The basic idea of IMAP is to store the messages on the server, so that they can be accessed on many different devices. It is possible to retrieve all messages to a client device and remove them from the server. Unlike an earlier alternative POP, IMAP does not force that.

IMAP scales well to different grades of connection, since messages and their attachments can be retrieved selectively.

IMAP allows messages to be moved between folders. In the e-mail server, it is possible to define sorting rules for incoming messages.

Nowadays, IMAP has been partly replaced with webmail services. There still are some unique benefits, such as a smaller resource usage. For instance, the server can notify the client of incoming messages, if the mail user agent (MUA) leaves the IMAP connection open and sends an IDLE command. The server would respond to it once a message arrives.

Messages are typically sent via a separate SMTP service.

The original IMAP was unencrypted, and an external attacker could eavesdrop connections for user names and passwords as well as the entire traffic in clear text. Nowadays, the TLS enhanced alternatives IMAPS and SMTPS are in use.

Access to mail boxes may be restricted to the virtual private network of a company or be opened to the public Internet.