TCP/IP Tutorial and Technical Overview - IBM Redbooks

– NOOP: Do nothing. The server returns a positive response.
– RSET: Cancel any previous delete commands.
– QUIT: Update the mailbox (delete any messages requested previously)
and then end the TCP connection.
15.5 Internet Message Access Protocol (IMAP4)
The Internet Message Access Protocol, Version 4 is an electronic messaging
protocol with both client and server functions. It is defined by RFC 3501. Similar
to POP, IMAP4 servers store messages for multiple users to be retrieved upon
client requests, but the IMAP4 model provides more functionality to users than
does the POP model. IMAP4 allows clients to have multiple remote mailboxes
from which messages can be retrieved, and allows users to choose any of those
at any point. IMAP4 clients can also specify criteria for downloading messages,
such as not transferring large messages over slow links. Additionally, IMAP4
always keeps messages on the server and replicates copies to the clients.
Another difference between POP and IMAP4 implementations is in the
operational mode. Using POP, a client must always be connected to the server
for changes to be made. However, IMAP4 allows clients to make changes both
when connected and when disconnected. When disconnected, (referred to as a
disconnected client), changes made on the client take effect on the server by
periodic re-synchronization of the client and server. Let us discuss the underlying
electronic mail models of IMAP4 first in order to understand the IMAP4 functions
clearly. These are described in detail in RFC 1733 – Distributed Electronic Mail
Models In IMAP4.
15.5.1 Fundamental IMAP4 electronic mail models
As defined in RFC 1733, there are three fundamental models implemented by
the IMAP4 client and server: offline, online, and disconnected.
The offline model is similar to POP3’s implementation (see 15.4, “Post Office
Protocol (POP)” on page 589). An IMAP4 client connects to a server, downloads
mail messages, and then disconnects from the server. Downloaded messages
are then deleted from the server-based mailbox and exist only on the client’s
system.
The online model is the opposite of the offline model. In the online model, an
IMAP4 client does not download messages from the server. Instead, it
establishes a connection with the server, and then manipulates the mail while it
remains in the server-based mailbox.
Chapter 15. Mail applications 591