See Internet Message Access Protocol Version 4.
Internet Mail Access Protocol version 4; standardised way of accessing a centrally stored part of a mailbox and combining it with a locally stored part. It allows synchronisation between locally and remotely stored folders.
Internet Message Access Protocol Version 4 (IMAP4) allows users to be disconnected from the main messaging system and still be able to process their mail. The IMAP specification allows for administrative control for these disconnected users and for the resynchronization of the users' message store once they reconnect to the messaging system.