3. State and Flow Diagram

Once the connection between client and server is established, an IMAP4rev2 connection is in one of four states. The initial state is identified in the server greeting. Most commands are only valid in certain states. It is a protocol error for the client to attempt a command while the connection is in an inappropriate state, and the server will respond with a BAD or NO command completion result.

3.1 Not Authenticated State

In the not authenticated state, the client MUST supply authentication credentials before most commands will be permitted. This state is entered when a connection starts unless the connection has been pre-authenticated.

3.2 Authenticated State

In the authenticated state, the client is authenticated and MUST select a mailbox to access before commands that affect messages will be permitted. This state is entered when a pre-authenticated connection starts, when acceptable authentication credentials have been provided, after an error in selecting a mailbox, or after a successful CLOSE or UNSELECT command.

3.3 Selected State

In a selected state, a mailbox has been selected to access. This state is entered when a mailbox has been successfully selected.

3.4 Logout State

In the logout state, the connection is being terminated. This state can be entered as a result of a client request (via the LOGOUT command) or by unilateral action on the part of either the client or the server.

If the client requests the logout state, the server MUST send an untagged BYE response and a tagged OK response to the LOGOUT command before the server closes the connection; and the client MUST read the tagged OK response to the LOGOUT command before the client closes the connection.

A server SHOULD NOT unilaterally close the connection without first sending an untagged BYE response that contains the reason for doing so. A client SHOULD NOT unilaterally close the connection; instead, it SHOULD issue a LOGOUT command.

State Transition Diagram

                  +----------------------+
                  |connection established|
                  +----------------------+
                             ||
                             \/
           +--------------------------------------+
           |          server greeting             |
           +--------------------------------------+
                     || (1)       || (2)        || (3)
                     \/           ||            ||
           +-----------------+    ||            ||
           |Not Authenticated|    ||            ||
           +-----------------+    ||            ||
            || (7)   || (4)       ||            ||
            ||       \/           \/            ||
            ||     +----------------+           ||
            ||     | Authenticated  |<=++       ||
            ||     +----------------+  ||       ||
            ||       || (7)   || (5)   || (6)   ||
            ||       ||       \/       ||       ||
            ||       ||    +--------+  ||       ||
            ||       ||    |Selected|==++       ||
            ||       ||    +--------+           ||
            ||       ||       || (7)            ||
            \/       \/       \/                \/
           +--------------------------------------+
           |               Logout                 |
           +--------------------------------------+
                             ||
                             \/
               +-------------------------------+
               |both sides close the connection|
               +-------------------------------+

Legend

• (1) connection without pre-authentication (OK greeting)
• (2) pre-authenticated connection (PREAUTH greeting)
• (3) rejected connection (BYE greeting)
• (4) successful LOGIN or AUTHENTICATE command
• (5) successful SELECT or EXAMINE command
• (6) CLOSE or UNSELECT command, unsolicited CLOSED response code, or failed SELECT or EXAMINE command
• (7) LOGOUT command, server shutdown, or connection closed

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