IETF RFC 2449 PDF

Introduction

The Post Office Protocol version 3 [POP3] is very widely used. However, while it includes some optional commands (and some useful protocol extensions have been published), it lacks a mechanism for advertising support for these extensions or for behavior variations.

Currently these optional features and extensions can only be detected by probing, if at all. This is at best inefficient, and possibly worse. As a result, some clients have manual configuration options for POP3 server capabilities.

Because one of the most important characteristics of POP3 is its simplicity, it is desirable that extensions be few in number (see section 7). However, some extensions are necessary (such as ones that provide improved security [POP-AUTH]), while others are very desirable in certain situations. In addition, a means for discovering server behavior is needed.

This memo updates RFC 1939 [POP3] to define a mechanism to announce support for optional commands, extensions, and unconditional server behavior. Included is an initial set of currently deployed capabilities which vary between server implementations, and several new capabilities (SASL, RESP-CODES, LOGIN-DELAY, PIPELINING, EXPIRE and IMPLEMENTATION). This document also extends POP3 error messages so that machine parsable codes can be provided to the client. An initial set of response codes is included. In addition, an [ABNF] specification of POP3 commands and responses is defined.