Using Telnet to Test SMTP Email Communication

Posted: March 3, 2016 in All, Miscellaneous

Recently we came across an issue where users complained about not receiving emails from our website. We have our own SMTP server setup for that website/application.
After verifying application logs as well as Pickup/Drop/Queue/BadMail SMTP folders on the server, everything looked fine. So, now the question is how to verify if the service is really sending emails or not. We might not be able to force the application to send a test email. In this scenario, we could test email functionality from command prompt using telnet. In this article, I put together the steps to send email using telnet.

To start with, we need to enable Telnet client on the machine from where we want to connect to SMTP service.

Steps to enable Telnet client:
Open Run window and type “appwiz.cpl”
It opens “Programs and Features” window.
Click on “Turn Windows features on or off” link from left pane.
“Windows Features” window will be opened.
Scroll down until you find “Telnet Client” option. Check the checkbox and say OK.
If it prompts for System restart, you can select “do not restart now” option because enabling Telnet Client option doesn’t require system reboot.

Testing email functionality:
Open command prompt.
Run the following commands to open telnet connection.

open <servername/IP> 25

25 is the default port number where SMTP service listens. Make sure your SMTP service is configured with default port number. If not use appropriate port number.
In the above command, we are opening telnet first and then opening a session with Server.
Instead we could use the following single command:

telnet <servername/IP> 25

If it is not able to establish a connection, then an error message would be displayed as “Could not open connection to the host…”. Then you need to check for connection error/firewall settings etc.

Assuming we successfully established the connection, the next step is to say hello to the server using following command. You would get a hello response from service.


Now, start creating an email. First enter from address using following command.
Note that there is a space between colon (:) and email address. After multiple attempts, I realized that space is mandatory.  Without a colon, it throws some random error and the error message doesn’t help much.

mail from:

Enter the recipient’s address using following command. Here space is not required after colon (I didn’t know why?)


Next step is to enter email text. To do this, type following command and press enter.


Then enter email Subject. Press “Enter” twice. Then enter email body/content. Then press “Enter” twice. In the end enter a period (.) and press “Enter”.

Subject:(email subject…)
(email content goes here… )

Once you press Enter, you would see a confirmation message saying the message was sent successfully. You can check your email and verify email headers. The headers should show the SMTP server name from where the message was sent.

While testing this functionality, I observed that sometimes it throws random errors though you enter correct commands.  I realized that if we re-execute the same command next time, it would be successful.



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