IMAP (Internet Message Access Protocol) is a protocol used to retrieve and store emails. More recent than POP, IMAP allows folders and rules on the server.
Unlike POP3, IMAP allows multiple simultaneous connections to one mailbox. Clients accessing a mailbox can receive information about state changes made from other clients. IMAP also provides a mode for clients to stay connected and receive information on demand.
Mark Crispin initially developed IMAP in 1986 as Interim Mail Access Protocol. IMAP4 revision 1 is the current version, defined by RFC 3501.