RFC 1225 (rfc1225) - Page 3 of 16
Post Office Protocol: Version 3
Alternative Format: Original Text Document
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: PostMaster@UDEL.EDU) 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), Rose
