Please Whitelist This Site?

I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :)

If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads.

If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||^$document". Then just click OK.

Thanks for your understanding!

Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide

NOTE: Using software to mass-download the site degrades the server and is prohibited.
If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The Book is Here... and Now On Sale!

The whole site in one document for easy reference!
The TCP/IP Guide

Custom Search

Table Of Contents  The TCP/IP Guide
 9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)
      9  TCP/IP Key Applications and Application Protocols
           9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)
                9  TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)
                     9  TCP/IP Electronic Mail Access and Retrieval Protocols and Methods
                          9  TCP/IP Post Office Protocol (POP/POP3)

Previous Topic/Section
POP Overview, History, Versions and Standards
Previous Page
Pages in Current Topic/Section
Next Page
POP3 Authorization State: User Authentication Process and Commands
Next Topic/Section

POP3 General Operation, Client/Server Communication and Session States
(Page 1 of 2)

As I mentioned in the previous topic, the Post Office Protocol was designed to perform only one main function, and to do it quickly and simply: allow a client computer to access and retrieve e-mail from a server. The operation of the protocol is simple and straight-forward, which is good news for you, the person trying to understand how it works.

Client/Server Communication

POP3 is a regular TCP/IP client/server protocol. To provide access to mailboxes, POP3 server software must be installed and continuously running on the server where the mailboxes are located. This does not necessarily have to be the same physical hardware device that runs the SMTP server software that receives mail for those boxes—a mechanism such as NFS may be used to allow the POP3 and SMTP servers to both “see” mailboxes locally. POP3 clients are regular end-user e-mail programs that make connections to POP3 servers to get mail; examples include Microsoft Outlook, Eudora Email and so forth.

POP3 uses the Transmission Control Protocol (TCP) for communication, to ensure the reliable transfer of commands, responses and message data. POP3 servers “listen” on well-known port number 110 for incoming connection requests from POP3 clients. After a TCP connection is established, the POP3 session is activated. The client sends commands to the server, which replies with responses and/or e-mail message contents.

Commands and Response Codes

POP3 commands are three or four letters long and are case-insensitive. They are all sent in plain ASCII text and terminated with a “CRLF” sequence, just as with FTP and SMTP commands. POP3 replies are also textual, but the protocol does not use the complex three-digit reply code mechanism of FTP (and SMTP). In fact, it defines only two basic responses:

  • +OK: A positive response, sent when a command or action is successful; and

  • -ERR: A negative response, sent to indicate that an error has occurred.

These messages may be accompanied by explanatory text, especially in the case of an -ERR response, to provide more information about the nature of the error.

Previous Topic/Section
POP Overview, History, Versions and Standards
Previous Page
Pages in Current Topic/Section
Next Page
POP3 Authorization State: User Authentication Process and Commands
Next Topic/Section

If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!
Donate $2
Donate $5
Donate $10
Donate $20
Donate $30
Donate: $

Home - Table Of Contents - Contact Us

The TCP/IP Guide (
Version 3.0 - Version Date: September 20, 2005

Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.