7.2 Using SMTP Commands to Troubleshoot and Test SMTP Servers

Before diving into specific software-configuration tips, here's a technique that can be used to troubleshoot or test any SMTP server: manual mail delivery. Normally, end users don't use SMTP commands because end users generally don't transfer their email manually. That's the job of MUAs, MDAs, and MTAs.

But it so happens that SMTP is a simple ASCII-based protocol built on TCP, and it's therefore possible to use SMTP commands to interact directly with an email server by telneting to TCP port 25 on that server. This is a useful technique for checking and troubleshooting MTA configurations. All you need is a telnet client and a working knowledge of a few of the commands in RFC 2821.

Here's an example session:

$ telnet buford.hackenbush.com 25
Trying 10.16.17.123...
Connected to buford.hackenbush.com.
Escape character is '^]'.
220 buford.hackenbush.com ESMTP Postfix
helo woofgang.dogpeople.org
250 buford.hackenbush.org
mail from:<[email protected]>
250 Ok
rcpt to:<[email protected]>
250 Ok
data
354 End data with <CR><LF>.<CR><LF>
Subject: Test email from Mick
Testing, testing, 1-2-3...
.
250 Ok: queued as F28B08603
quit
221 Bye
Connection closed by foreign host.

Let's dissect the example, one command at a time:

helo woofgang.dogpeople.org

The HELO command (SMTP commands are case insensitive) provides the remote server with your hostname or domain name.

mail from:<[email protected]>

The MAIL command is used to specify your email's "from:" address. Again, this is usually taken at face value.

rcpt to:<[email protected]>

Use the RCPT command to specify your email's "to:" address. This address may or may not be validated: a well-configured SMTP host will reject nonlocal destination addresses for incoming mail to prevent unauthorized mail relaying.

data

DATA means "and now, here's the message." To specify an optional Subject line, make the first word of the first line of your message "Subject:", followed immediately by your subject string. You can specify other SMTP headers too, each on its own line; if you want, you can even make headers up — e.g., "X-Slartibartfast: Whee!"

When your message is complete, type a period on an empty line, and press RETURN.

quit

QUIT closes the SMTP session.

My own procedure to test any SMTP server I set up is first to deliver a message this way from the server to itself — i.e., telnet localhost 25. If that succeeds, I then try the same thing from a remote system.

This technique doesn't work for advanced setups like SMTP over TLS (covered later in this chapter), but it's a fast, simple, and reliable test for basic SMTP server configurations, especially when you need to verify that antirelaying and other controls have been set correctly.