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 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 System Overview and Concepts
TCP/IP Electronic Mail Message Communication Model and Device and Protocol Roles
(Page 3 of 3)
Protocol Roles In Electronic Mail Communication
You may have noticed that SMTP is
used for most of this communication process. In fact, if the recipient
uses a machine that runs SMTP software, which is common for those using
dialup UNIX shell Internet access, the process of sending e-mail uses
SMTP exclusively. SMTP servers must, however, always be available on
the Internet and ready to accept mail. Most people access the internetwork
using devices that aren't always online or that don't run SMTP software.
That is why the last step, mail access and retrieval, is usually required.
It might have been possible to define
the electronic mail system so that this last step of communication was
carried out using SMTP as well, which would mean the entire system used
the same protocol. However, SMTP was tailored for the specific purpose
of transporting and delivering e-mail, not for remote mailbox access.
It made more sense to leave the function of mailbox access to dedicated,
separate protocols. This not only allows these protocols to be tailored
to the needs of e-mail recipients, but provides flexibility by giving
users more than one option for how e-mail is retrieved. I discuss mail
access protocols and methods in a separate
section of the Guide, highlighting the
two most common protocols: the Post
Office Protocol (POP) and the Internet
Message Access Protocol (IMAP).
The three protocols discussed aboveSMTP,
POP3 and IMAPget the lead billing on the TCP/IP electronic
mail stage, but they rely on two other elements to play supporting
roles. The first is a method
of addressing e-mail messages to ensure
that they arrive at their destinations. The second is the set of message
formats used to encode messages and control
how they are delivered and used. These don't usually get as much attention
as they deserve, but they do here, as I have devoted the next two sections
Key Concept: One of the critical requirements of an electronic mail system is that the sender and receiver of a message not be required to both be on the system at the time mail is sent. TCP/IP therefore uses a communication model with several devices that allow the sender and recipient to be decoupled. The senders client device spools mail and moves it to the senders local SMTP server when it is ready for transmission; the e-mail is then transmitted to the receivers SMTP server using SMTP. The e-mail can remain on the recipients server for an indefinite period of time. When the recipient is ready to read it, he or she retrieves it using one or more of a set of mail access protocols and methods, the two most popular of which are POP and IMAP.
|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!|
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.