POP3 Post Office Protocol-version 3

1. Introduction

For smaller nodes on the network, it is impractical to support message transfer systems (MTS). For example, a workstation may not have sufficient resources to allow the SMTP server and equivalent local messaging system to stay in order and continue to run. Similarly, the cost of connecting a personal computer over a long period of time on an IP-type network is also considerable (the resources missing from the node are called "Liaisons").

Nonetheless, it is useful to allow management of messages at such a summary point, and these nodes often support a user agent to manage messages. To solve this problem, the ability to support MTS nodes provides a message storage function for these unsupported nodes. Post Office Protocol-version 3 makes it possible for such workstations to use a more practical method for accessing stored messages stored on the server. Typically, this means that the workstation can get the message from the server, and the server temporarily saves the message for it.

In the following, the client host refers to the host that utilizes the POP3 service, and the server host refers to the host that provides the POP3 service.

2. Simple description

This document does not indicate how the client host will send the message to the delivery system. But here's a note: When a user agent needs to send information to the delivery system, it establishes an SMTP connection on the relay host (these relay hosts can be POP3 or not).

3. Basic operation

Initially, the server starts the POP3 service by listening on TCP port 110. When a client host needs to use a service, it establishes a TCP connection with the server host. When the connection is established, POP3 sends a confirmation message. The client and the POP3 Server Exchange commands and responses to each other (respectively), and this process continues until the connection terminates.

The POP3 command consists of a command and some parameters. All commands with a CRLF to the end. Commands and parameters are made up of printable ASCII characters, separated by spaces. Commands are typically three to four letters, and each argument can be up to 40 characters in length.

The POP3 response consists of a status code and a command that may have additional information. All responses are also ended by CRLF pairs. There are now two status codes, "OK" ("+ok") and "Fail" ("-err").