IMAP is a standard IETF protocol to access mail folders stored on a remote mail server. Unlike POP3 which gives only limited access to a single INBOX, IMAP permits manipulation of a remote mail store in a way that is functionally equivalent to local mailboxes.

Unlike many common IETF protocols, IMAP is not a one-request/one-response protocol. The client might issue one or more actions to be performed in parallel, thus responses to those commands can arrive in an order independent from the order they were issued. Requests and the appropriate responses are paired by a unique request identifier called 'tag'. There is one exception to this rule: the server might return untagged responses, when more than a single response is associated with a single command. In this case the server responds with one or more untagged responses and at the end a tagged response to indicate the end of the processing.