RFC 1081 (rfc1081) - Page 2 of 16
Post Office Protocol: Version 3
Alternative Format: Original Text Document
RFC 1081 POP3 November 1988 For the remainder of this memo, the term "client host" refers to a host making use of the POP3 service, while the term "server host" refers to a host which offers the POP3 service. A Short Digression This memo does not specify how a client host enters mail into the transport system, although a method consistent with the philosophy of this memo is presented here: When the user agent on a client host wishes to enter a message into the transport system, it establishes an SMTP connection to its relay host (this relay host could be, but need not be, the POP3 server host for the client host). If this method is followed, then the client host appears to the MTS as a user agent, and should NOT be regarded as a "trusted" MTS entity in any sense whatsoever. This concept, along with the role of the POP3 as a part of a split-UA model is discussed later in this memo. Initially, the server host starts the POP3 service by listening on TCP port 110. When a client host wishes to make use of the service, it establishes a TCP connection with the server host. When the connection is established, the POP3 server sends a greeting. The client and POP3 server then exchange commands and responses (respectively) until the connection is closed or aborted. Commands in the POP3 consist of a keyword possibly followed by an argument. All commands are terminated by a CRLF pair. Responses in the POP3 consist of a success indicator and a keyword possibly followed by additional information. All responses are terminated by a CRLF pair. There are currently two success indicators: positive ("+OK") and negative ("-ERR"). Responses to certain commands are multi-line. In these cases, which are clearly indicated below, after sending the first line of the response and a CRLF, any additional lines are sent, each terminated by a CRLF pair. When all lines of the response have been sent, a final line is sent, consisting of a termination octet (decimal code 046, ".") and a CRLF pair. If any line of the multi-line response begins with the termination octet, the line is "byte-stuffed" by pre-pending the termination octet to that line of the response. 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 Rose
