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 tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$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!

Get The TCP/IP Guide for your own computer.
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
POP3 General Operation, Client/Server Communication and Session States
Previous Page
Pages in Current Topic/Section
1
2
3
Next Page
POP3 Transaction State: Mail and Information Exchange Process and Commands
Next Topic/Section

POP3 Authorization State: User Authentication Process and Commands
(Page 2 of 3)

Standard POP3 Authentication

The normal method of authorization in POP3 is a standard “username / password” login. This is pretty much identical to how a login is performed in FTP; even the commands are the same. First the client issues a USER command along with the user's mailbox name (his or her user name or e-mail address). The server responds with an intermediate acknowledgment. The client then uses the PASS command to send the user's password. Assuming the login is valid, the server responds to the client with an acknowledgment that indicates successful authentication. The response will also typically specify the number of messages waiting for the user in the mailbox. This process is illustrated in Figure 307.


Figure 307: Post Office Protocol (POP3) User Authentication Process

Once the TCP connection is established from the client to the server, the server responds with a greeting message, and the simple POP3 authentication process begins. The client sends a user name and password to the server using the USER and PASS commands, and the server evaluates the information to determine whether or not it will allow the client access.

 


Table 256 shows an example POP3 authorization, with the client's commands highlighted and the server's responses in italics.


Table 256: Example POP3 Authorization

+OK POP3 server ready
USER jane@somewhereelse.com
+OK
PASS *******
+OK jane@somewhereelse.com has 3 messages


Note: Some servers may require only the name of the user (“jane”) while others require the full e-mail address as shown in the example. Also, I have not shown Jane's password in Table 256; that wouldn't be very nice, would it? J


If authorization is successful, the POP3 session transitions to the Transaction state where mail access commands can be performed. If the user name or password are incorrect, an error response is given and the session cannot proceed. The authorization may also fail due to technical problems, such as an inability by the server to lock the mailbox (perhaps due to new mail arriving via SMTP).


Previous Topic/Section
POP3 General Operation, Client/Server Communication and Session States
Previous Page
Pages in Current Topic/Section
1
2
3
Next Page
POP3 Transaction State: Mail and Information Exchange 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 (http://www.TCPIPGuide.com)
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.