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!

Searchable, convenient, complete TCP/IP information.
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 Delivery Protocol: The Simple Mail Transfer Protocol (SMTP)

Previous Topic/Section
SMTP Overview, History and Standards
Previous Page
Pages in Current Topic/Section
1
2
3
Next Page
SMTP Connection and Session Establishment and Termination
Next Topic/Section

SMTP Communication and Message Transport Methods, Client/Server Roles and Terminology
(Page 2 of 3)

Modern E-Mail Communication Using DNS and Direct Delivery

The creation of DNS radically changed how e-mail delivery worked. DNS includes support for a special mail exchanger (MX) record that allows easy mapping from the domain name in an e-mail address to the IP address of the SMTP server that handles mail for that domain. I explain this in the topic on the regular e-mail address format, as well as the dedicated topic on DNS e-mail support.

In the new system, SMTP communication is much simpler and more direct. The sending SMTP server uses DNS to find the MX record of the domain to which the e-mail is addressed. This gives the sender the DNS name of the recipient's SMTP server. This is resolved to an IP address, and a connection can be made directly from the sender's SMTP server to the recipient's to deliver the e-mail. While SMTP still supports relaying, direct e-mail delivery using MX records is faster and more efficient, and RFC 2821 makes clear that this is now the preferred method.

In this new system, SMTP is generally only used for two transfers: first, from the sender's client machine to the sender's local SMTP server, and then from that server to the recipient's local SMTP server, as shown in Figure 301. (A distinct mail access protocol or method is used by the recipient for the last leg of the journey.) Each transfer of an e-mail message between SMTP servers involves the establishment of a TCP connection and then the transfer of the e-mail headers and body using the SMTP mail transfer process. The next two topics describe in detail how this occurs.

Key Concept: In the early days of SMTP, mail was delivered using the relatively inefficient process of relaying from server to server across the internetwork. Today, when an SMTP server has mail to deliver to a user, it determines the server that handles the user’s mail using the Domain Name System (DNS) and sends the mail to that server directly.



Previous Topic/Section
SMTP Overview, History and Standards
Previous Page
Pages in Current Topic/Section
1
2
3
Next Page
SMTP Connection and Session Establishment and Termination
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.