Test an SMTP server from commandline using telnet

Introduction

Whenever I look at protocols I always get positively surprised at how easy they are to understand once you take an actual look at it.

This time around I wanted to test an SMTP server without the need to set up some random test client software or putting something together in Python or Node.js. This time, I wanted something plain simple.

telnet

telnet is a tool that is very handy in talking with servers. No magic goes on under the hood. Perform simple conversations from a client to a server.

To connect to a server on the default plain-text SMTP port (25) you can use telnet target.host.example.com smtp and start the conversation.

Below is an example transmission of sending an email over SMTP. All lines starting with three numbers are responses from the server. HELO, MAIL, RCPT, DATA, and QUIT are commands written and sent from the client to the server.

$ telnet mail.example.com smtp
Trying 127.0.0.1...
Connected to localhost.localdomain (127.0.0.1).
Escape character is '^]'.
220 mail.example.com ESMTP Sendmail 8.13.8/8.13.8; Tue, 22 Oct 2013 05:05:59 -0400
HELO myserver.no
250 myclient.hostname.example.no Hello myclient.hostname.example.no [127.0.0.1], pleased to meet you
MAIL from: sender@myclient.hostname.example.no
250 2.1.0 sender@myclient.hostname.example.no... Sender ok
RCPT to: mom@example.com
250 2.1.5 mom@example.com... Recipient ok
DATA
354 Enter mail, end with "." on a line by itself
Hey
This is test email only

Thanks
.
250 2.0.0 r9M95xgc014513 Message accepted for delivery
QUIT
221 2.0.0 mail.example.com closing connection
Connection closed by foreign host.

The same as above with server replies and telnet output omitted:

HELO myserver.no
MAIL from: sender@myclient.hostname.example.no
RCPT to: mom@example.com
DATA
Hey
This is test email only

Thanks
.
QUIT

Now it’s easy to see how simple the SMTP protocol is.

References

• https://tecadmin.net/ways-to-send-email-from-linux-command-line/