IMAP (Internet Message Access Protocol) is a method of accessing electronic mail or bulletin board messages that are kept on a (possibly shared) mail server.

In other words, it permits an email program (or mail client) to access remote message stores as if they were local.

For example, email stored on an IMAP server can be manipulated from a desktop computer at home, a workstation at the office, and a notebook computer while travelling, without the need to transfer messages or files back and forth between these computers.

IMAP's ability to access messages (both new and saved) from more than one computer has become extremely important as reliance on electronic messaging and use of multiple computers increase.

However, this functionality should not be taken for granted: the widely used Post Office Protocol (POP3) works best when one has only a single computer, since it was designed to support "offline" message access, wherein messages are downloaded and then deleted from the mail server.

Because e-mail retrieved using IMAP is not downloaded to the local computer, but rather cached, and kept stored on the remote computer, it is necessary to be connected to the internet in order to read messages.