RFC1939-POP3 protocol

1. Introduction
For small nodes on the network, it is impractical to support the message Transmission System (MTS).
. For example, a workstation may not have sufficient resources to allow the SMTP server and a considerable number of local email delivery systems to stay in sequence and run continuously. Similarly, the cost of connecting a personal computer to an IP network over a long period of time is considerable (the lack of resources at the node is called "contact ").
Even so, it is very useful to allow mail management on such summary points, and these nodes often support a user agent to manage mail. To solve this problem, the nodes that support MTS provide the mail storage function for those nodes that cannot be supported. The Post Office Protocol-Version 3 enables such workstations to use a more practical method to access stored mails stored on servers. Generally, this means that the workstation can get the mail from the server, and the server temporarily saves the mail for it.
In the following section, a client host refers to a host that uses the POP3 service, and a server host refers to a host that provides the POP3 service.
2. Simple description
This document does not specify how the client host sends messages to the transfer system. But here is a description: when the user agent needs to send the information to the transfer system, it establishes an SMTP connection on the relay host (these can be POP3 hosts, or not ).

3. Basic operations
Initially, the server listens to TCP port 110 to start the POP3 service. When the customer host needs to use the service, it establishes a TCP connection with the server host. After the connection is established, POP3 sends a confirmation message. The client and POP3 server exchange commands and responses to each other (respectively). This process continues until the connection is terminated.
The POP3 command consists of a command and some parameters. All commands end with a CRLF pair. Commands and parameters are composed of printable ASCII characters separated by spaces. The command generally contains three to four letters, but each parameter can be up to 40 characters long.
A POP3 response consists of a status code and a command that may contain additional information. All responses are also ended by the CRLF pair. There are two status codes: "OK" ("+ OK") and "failed" ("-ERR ").
The response to a specific command is composed of many characters. In these cases, the following statements are one by one: after the first line of response and a CRLF are sent, any additional information lines are sent, and they are also ended by the CRLF pair. When all messages are sent, the last line is sent, including an ending character (Decimal Code 46, that is, ".") and a CRLF pair. If any row in the information starts with an ending character, this row is filled by preloading and ending the row. Therefore, the multi-line response is ended by five CRLF. CRLF. When multiple lines are detected, the customer checks to confirm whether the line starts with an ending character. If yes, and the subsequent character is not CRLF, the first character (ending character) of this line will be discarded; if it is followed by CRLF, the response from the POP server will terminate, including. CRLF rows are not considered part of multiline responses.