Introduction

Although modern email systems primarily use authenticated SMTP clients and APIs, Telnet remains a useful tool for testing SMTP connectivity and diagnosing mail server issues. System administrators and DevOps engineers often use Telnet to verify whether an SMTP server accepts connections and processes email commands correctly.

This guide explains how to send a test email using Telnet from a remote Linux server.

Prerequisites

Before starting, ensure that:

  • Telnet is installed on the remote server.
  • The SMTP server is reachable.
  • Port 25, 587, or 465 is open (depending on your mail server configuration).
  • The SMTP server allows connections from your server.

To install Telnet:

Ubuntu/Debian

sudo apt update
sudo apt install telnet -y

RHEL/CentOS

sudo yum install telnet -y

Step 1: Connect to the SMTP Server

telnet smtp.example.com 25

Example output:

Trying 192.168.1.100...
Connected to smtp.example.com.
220 smtp.example.com ESMTP Postfix

Step 2: Introduce Your Server

HELO remote.example.com

Response:

250 smtp.example.com

Step 3: Specify the Sender

MAIL FROM:<sender@example.com>

Expected response:

250 2.1.0 Ok

Step 4: Specify the Recipient

RCPT TO:<recipient@example.com>

Expected response:

250 2.1.5 Ok

Step 5: Start the Email Body

DATA

Response:

354 End data with <CR><LF>.<CR><LF>

Now enter the email:

Subject: SMTP Test Email

Hello,

This is a test email sent using Telnet from a remote Linux server.

Regards,
System Administrator
.

The period (.) on a separate line indicates the end of the message.

Expected response:

250 2.0.0 Ok: queued as 8A3F112345

Step 6: Close the Connection

QUIT

Response:

221 2.0.0 Bye

Complete Example

telnet smtp.example.com 25

HELO remote.example.com
MAIL FROM:<sender@example.com>
RCPT TO:<recipient@example.com>
DATA
Subject: SMTP Test

Hello,
This email was sent using Telnet.

Thank you.
.
QUIT

Troubleshooting

Connection Refused

telnet: Unable to connect to remote host

Check:

  • SMTP service is running.
  • Firewall rules.
  • Security groups.
  • Network routing.

Relay Access Denied

554 Relay access denied

This means your SMTP server does not allow unauthenticated relay. Configure SMTP authentication or whitelist the server.

Connection Timeout

Connection timed out

Verify:

  • Port accessibility.
  • Firewall configuration.
  • ISP restrictions on SMTP traffic.

Testing SMTP Connectivity

To verify connectivity before sending an email:

telnet smtp.example.com 25

or

nc -vz smtp.example.com 25

Example:

Connection to smtp.example.com 25 port [tcp/smtp] succeeded!

Security Considerations

  • Telnet sends data in plain text.
  • Avoid using Telnet on production SMTP servers requiring authentication.
  • For secure email transmission, use SMTP over TLS (port 587) or SMTPS (port 465).
  • Use Telnet primarily for troubleshooting and connectivity testing.

Conclusion

Sending email through Telnet is a simple yet powerful method for testing SMTP servers. It helps administrators verify mail server connectivity, identify relay issues, troubleshoot firewall problems, and confirm SMTP functionality without relying on email clients. While it is not intended for production email delivery, Telnet remains an essential diagnostic tool for system administrators

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