The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)                 9  TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)                      9  TCP/IP Electronic Mail Access and Retrieval Protocols and Methods                           9  TCP/IP Internet Message Access Protocol (IMAP/IMAP4)

IMAP Overview, History, Versions and Standards 1 2 3 IMAP Commands, Results and Responses

Normal Session Establishment and Greeting

The server determines in which state the IMAP session begins, and sends a greeting message to tell the client the session is established and indicate which state it is in. Normally, the server will begin the session in the Not Authenticated state. This is conveyed to the client with the normal OK greeting message, such as this:

* OK <server-name> server ready

In certain circumstances, a server may already know the identity of the client, perhaps as a result of some external authentication mechanism not part of the IMAP protocol. In this case, a special greeting is used:

* PREAUTH <server-name> server ready, logged in as <user-name>

This tells the client that it is already in the Authenticated state.

If the server decides for whatever reason not to accept a new session from the client, it can respond with a BYE response instead of OK or PREAUTH, and close the TCP connection.

IMAP Overview, History, Versions and Standards 1 2 3 IMAP Commands, Results and Responses