Internet Messaging Access Protocol 4

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In POP3, once a message has been downloaded from the server, it is, by default, deleted from the server. This deletion is a real disadvantage for users who move from workstation to workstation because mail they have already downloaded remains on the workstation to which they downloaded it. IMAP4 was developed to allow users to leave their mail on the server and to allow remote access to messages. Thus, IMAP4 extends the functionality of POP3 to allow both offline and online storage of messages.

In addition, IMAP4 allows user-initiated storage of messages and nonmail messages, permits users to manage their own configurations, and allows the sharing of mailboxes. This protocol allows a client to manipulate mail messages on a server as though it were a local mailbox, unlike POP3, which can do little more than copy a message from a POP3 server to a local mailbox.

When a client connects to an IMAP4 server, it does so over TCP port 143. The IMAP4 server is always in one of four states. For each state, the client can issue a limited number of commands to the server. Some commands transition the server into the next state. It is a protocol error for the client to issue a command that is not appropriate to the present state of the server. Figure 17-14 shows the IMAP4 states for an IMAP4 server as they are described in RFC 2060. Table 17-4 lists the more common IMAP4 commands.

Figure 17-14. IMAP4 states as described in RFC 2060.

Table 17-4. IMAP4 commands

Command	Description
CAPABILITY	Requests a listing of the functionality of the server
AUTHENTICATE	Indicates an authentication mechanism
LOGIN	Identifies a client with user name and password
SELECT	Selects the mailbox to use
EXAMINE	Selects a mailbox in read-only mode
CREATE	Creates a mailbox
DELETE	Deletes a mailbox
RENAME	Renames a mailbox
SUBSCRIBE	Adds a mailbox to the server's set of active mailboxes
UNSUBSCRIBE	Removes a mailbox from the server's set of active mailboxes
LIST	Lists a set or subset of mailboxes
LSUB	Lists subscribed mailboxes
STATUS	Requests the status of a mailbox
APPEND	Adds a message to a mailbox
CLOSE	Effects pending deletions and closes a mailbox
EXPUNGE	Effects pending deletions
SEARCH	Searches a mailbox for messages satisfying a given criterion
FETCH	Fetches specified body parts for a given message
STORE	Changes the data of specified messages in a mailbox
COPY	Copies a message to another mailbox
NOOP	No action required
LOGOUT	Closes the connection

Administering IMAP4

IMAP4 is, for the most part, self-administering. You may want to consider a couple of items, however. Figure 17-15 shows the General tab of the property sheet for the IMAP4 default virtual server. Because IMAP4 has the ability to request public folders, Microsoft's implementation of this protocol lets you choose whether to show public folders to the client. In addition, you can enable fast message retrieval here, which will cause Exchange Server to approximate the message sizes, rather than calculate their exact size. This estimation is done only if the clients do not need to know the exact message sizes for retrieval.

Figure 17-15. IMAP4 default virtual server properties.