RFC 1225 (rfc1225) - Page 3 of 16

Post Office Protocol: Version 3

RFC 1225                          POP3                          May 1991


   Hence a multi-line response is terminated with the five octets
   "CRLF.CRLF".  When examining a multi-line response, the client checks
   to see if the line begins with the termination octet.  If so and if
   octets other than CRLF follow, the the first octet of the line (the
   termination octet) is stripped away.  If so and if CRLF immediately
   follows the termination character, then the response from the POP
   server is ended and the line containing ".CRLF" is not considered
   part of the multi-line response.

   A POP3 session progresses through a number of states during its
   lifetime.  Once the TCP connection has been opened and the POP3
   server has sent the greeting, the session enters the AUTHORIZATION
   state.  In this state, the client must identify itself to the POP3
   server.  Once the client has successfully done this, the server
   acquires resources associated with the client's maildrop, and the
   session enters the TRANSACTION state.  In this state, the client
   requests actions on the part of the POP3 server.  When the client has
   finished its transactions, the session enters the UPDATE state.  In
   this state, the POP3 server releases any resources acquired during
   the TRANSACTION state and says goodbye.  The TCP connection is then
   closed.

The AUTHORIZATION State

   Once the TCP connection has been opened by a POP3 client, the POP3
   server issues a one line greeting.  This can be any string terminated
   by CRLF.  An example might be:

      S.  +OK dewey POP3 server ready (Comments to: )

   Note that this greeting is a POP3 reply.  The POP3 server should
   always give a positive response as the greeting.

   The POP3 session is now in the AUTHORIZATION state.  The client must
   now issue the USER command.  If the POP3 server responds with a
   positive success indicator ("+OK"), then the client may issue either
   the PASS command to complete the authorization, or the QUIT command
   to terminate the POP3 session.  If the POP3 server responds with a
   negative success indicator ("-ERR") to the USER command, then the
   client may either issue a new USER command or may issue the QUIT
   command.

   When the client issues the PASS command, the POP3 server uses the
   argument pair from the USER and PASS commands to determine if the
   client should be given access to the appropriate maildrop.  If so,
   the POP3 server then acquires an exclusive-access lock on the
   maildrop.  If the lock is successfully acquired, the POP3 server
   parses the maildrop into individual messages (read note below),



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