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 Delivery Protocol: The Simple Mail Transfer Protocol (SMTP)

Previous Topic/Section
SMTP Communication and Message Transport Methods, Client/Server Roles and Terminology
Previous Page
Pages in Current Topic/Section
1
234
Next Page
SMTP Mail Transaction Process
Next Topic/Section

SMTP Connection and Session Establishment and Termination
(Page 1 of 4)

The delivery of electronic mail using the Simple Mail Transfer Protocol (SMTP) involves the regular exchange of e-mail messages between SMTP servers. SMTP servers are responsible for sending e-mail that users of the server submit for delivery. They also receive e-mail either intended for local recipients, or in some cases for forwarding or relaying to other servers.

Overview of Connection Establishment and Termination

All SMTP communication is done using the TCP. This allows SMTP servers to make use of TCP's many features that ensure efficient and reliable communication. SMTP servers generally must be kept running and connected to the Internet 24 hours a day, seven days a week, to ensure that mail can be delivered at any time. (This is a big reason why most end-users employ access protocols such as POP3 to access their received e-mail rather than running their own SMTP servers.) The server listens continuously on the SMTP server port, well-known port number 25, for any TCP connection requests from other SMTP servers.

As explained in the previous topic, an SMTP server that wishes to send e-mail normally begins with a DNS lookup of the MX record corresponding to the domain name of the intended recipient's e-mail address, to get the name of the appropriate SMTP server. This name is then resolved to an IP address; for efficiency, this IP address is often included as an Additional record in the response to the MX request, to save the sending server from needing to perform two explicit DNS resolutions.

The SMTP sender then establishes a SMTP session with the SMTP receiver. Once the session is established, mail transactions can be performed, to allow mail to be sent between the devices. When the SMTP sender is done, it terminates the connection. All of these processes involve specific exchanges of commands and replies, which are illustrated in Figure 304.


Figure 304: SMTP Transaction Session Establishment and Termination

An SMTP session begins with the SMTP sender establishing a TCP connection to the SMTP receiver. The receiver sends a ready message; the sender sends a HELO or EHLO command, to which the receiver responds. Assuming no difficulties, the session is established and mail transactions take place. When the sender is done, it sends a QUIT command; the receiver responds with a 221 reply and closes the session.

 


Previous Topic/Section
SMTP Communication and Message Transport Methods, Client/Server Roles and Terminology
Previous Page
Pages in Current Topic/Section
1
234
Next Page
SMTP Mail Transaction Process
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.