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 Security Issues
Previous Page
Pages in Current Topic/Section
1
23
Next Page
SMTP Replies and Reply Codes
Next Topic/Section

SMTP Commands
(Page 1 of 3)

I mentioned in the overview and history of SMTP that early TCP/IP e-mail mechanisms were developed by borrowing techniques and elements from existing application protocols, especially Telnet and FTP. SMTP is an independent protocol, but its heritage can still be seen clearly in a few areas. One of the more obvious of these is in the method by which commands are issued by an SMTP sender and replies returned by an SMTP receiver.

Like FTP, all SMTP commands are sent as plain ASCII text over the TCP connection established between the client and server in an SMTP connection. These commands must end with the two-character “CRLF” sequence that normally terminates ASCII text as required for the Telnet Network Virtual Terminal (NVT). In fact, you can check the function of an SMTP server and even issue commands to it yourself, simply by using Telnet to connect to it on port 25.

SMTP Command Syntax

All SMTP commands are specified using a four-letter command code. Some commands also either allow or require parameters to be specified. The basic syntax of a command is:

<command-code> <parameters>

When parameters are used, they follow the command code and are separated from it by one or more space characters. For example, the HELO and EHLO commands are specified with the command code, a space character, and then the domain name of the SMTP sender, as we saw in our look at SMTP connection establishment.


Previous Topic/Section
SMTP Security Issues
Previous Page
Pages in Current Topic/Section
1
23
Next Page
SMTP Replies and Reply Codes
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.