In this article, you will learn what SMTP commands are and how to use them to test SMTP issues such as basic SMTP health, re-laying checking, SMTP versions, etc.

If we send or receive mail over the internet or using Exchange 2000 or higher, it was transported via SMTP. Knowing more about SMTP and how to troubleshoot SMTP on a basic level is essential. Tools for the job are knowledge of the command syntax and a remote terminal tool such as Telnet.

Let’s start by Telneting to port 25 of a mail host and see what comes out of it. The command syntax will be

Telnet hostname 25

If you’re running a flavor of Exchange, you may receive a reply similar to:

220 win2k3dc.target.local Microsoft ESMTP MAIL Service, Version: 6.0.3790.0 ready at  Thu, 31 Mar 2005 11:35:01 +0200

I’m going to break the reply down and explain each part:

220

A status code implying that Exchange is available and listening. SMTP mail systems have to return a status code for each line of every mail conversation, which indicates anything from command received successfully, to “go away – you’re not wanted here”. Some replies may only be numbers; others may be numbers with an explanatory text. At the end of this article I have listed some more detail about the meaning of these.

win2k3dc.target.local

The hostname responsible for the service

Microsoft ESMTP MAIL Service, Version: 6.0.3790

Software responding to the service request:

ready at  Thu, 31 Mar 2005 11:35:01 +0200

So what do we do now? How do we know what to type? SMTP conversations mirror human conversations in that humans greet, exchange pleasantries that help establish who we are, talk and then say goodbye. For a successful SMTP conversation to occur, something similar happens, and some of the words used may look familiar.

In SMTP talk, that may look like this:

Notice the first word after the 220 message was HELO. This could also have been EHLO. For the purposes of this article, SMTP can be presented in two basic flavors, SMTP and ESMTP. ESMTP is all that SMTP is, with some extra bits bolted on, like large message support, pipelining, authentication etc, which makes it a perfect candidate for Microsoft to build a transport mechanism around, which in fact is what they did. Most internal server to server messages originate and are received using ESMTP, not SMTP. Entering EHLO instead of HELO will produce a list of available commands that Exchange will accept, along with a “ready” message.

Each line was responded to by codes starting with a 2xx or a 5xx. Either my input was good 2xx or I entered a command that was flawed or unacceptable 5xx.

In this case, even though I’m trying to impersonate “noddy.com” my real domain is echoed back to me on the next line as: 250 win2k3dc.target.local Hello [127.0.0.1]