Internet Message Access Protocol, v.4 (IMAP4)
Internet Message Access Protocol, version 4 is a more complex protocol, which provides more extensive functionality than is available through POP3. With IMAP4, clients can not only retrieve messages from a server, but they can also manipulate the remote message folders (or mailboxes) in which the messages are stored. Whereas POP3 simply allows messages to be downloaded to a local mailbox, IMAP4 includes operations for creating, deleting and removing remote mailboxes as well as for checking for new messages and selective fetching of message attributes. Of particular appeal to many users is the fact that it also provides facilities for offline support and synchronization.
The formal definition of the IMAP4 protocol can be found in RFC 1730.