Hack into Sending Email via Telnet

Ever wanted to step into the matrix and directly control the SMTP protocol? Telnet lets you manually send email at the command line for diagnostics and troubleshooting. Learn to talk SMTP like a wizard!

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An Introduction to Sending Email via Telnet

The Telnet protocol has been around since the early days of the internet. Developed in 1969 as part of the US government’s ARPANET project, it allowed users to establish connections with and send commands to remote computers.

While newer and more secure protocols like SSH have largely replaced Telnet for remote administration, it still remains a handy tool for manually testing low-level network communications. This includes verifying the SMTP protocol used for sending email.

A Brief History of the Telnet Protocol

Telnet dates all the way back to the late 1960s, when the protocol was standardized in RFC 15. It was originally developed as part of ARPANET, the US Defense Department’s early packet switching network that was the foundation of today’s internet.

The name “Telnet” is derived from “Teletype Network”, reflecting its initial purpose of facilitating connections between Teletype terminal devices across a network.

Throughout the 1970s and 1980s, Telnet gained widespread popularity as the protocol for remotely accessing multi-user systems like mainframes and UNIX servers. It became a standard tool for connectivity, allowing users to log into text-based interfaces on remote systems.

However, Telnet had some drawbacks. Namely, it transmitted all data including usernames and passwords in clear text. This meant communications could be intercepted and read. As more secure alternatives like SSH emerged in the 1990s, Telnet usage began to decline.

Still, the protocol remains useful today in certain cases. It provides a simple way to manually test network services at the most basic level.

Using Telnet to Send SMTP Email Commands

One of Telnet’s enduring uses is testing SMTP services by hand. SMTP, or Simple Mail Transfer Protocol, is the standard protocol used for sending email across the internet.

Here’s the gist of how Telnet enables manual SMTP testing:

  • Telnet allows you to connect to an SMTP server and communicate directly with it.
  • You send SMTP commands and the server returns responses, just like in an actual email sending conversation.
  • Using this manual approach, you can diagnose mail flow issues or test configurations.

For example, you can use Telnet to:

  • Verify that you can reach a remote SMTP server.
  • Send test emails through relays to check configurations.
  • Manually walk through the SMTP protocol steps to see where problems occur.
  • Spot errors in SMTP responses for troubleshooting.

Though Telnet is all text-based, you are essentially emulating an email client’s sending functions under the hood. Except you get direct control over the SMTP protocol instead of having a client library handle everything behind the scenes.

Benefits of Testing SMTP with Telnet

There are a few advantages to manually testing SMTP with Telnet versus automated testing tools:

Direct Control and Visibility

With Telnet, you can send the SMTP commands yourself and view the raw server responses. This gives you deep visibility into the conversation. You can tweak parameters, re-run tests, and thoroughly diagnose issues.

No Special Software Needed

Telnet is built into most operating systems already. So no need to install or configure a special SMTP testing tool. Just use the Telnet client you likely have handy.

Helps Learn SMTP Internals

Stepping through SMTP commands manually helps develop a solid understanding of how the protocol really works under the hood. Useful knowledge for any email admin or developer.

Can Test from Remote Locations

Your local computer might not exhibit the same connectivity or mail flow issues as a remote server. Testing from different network environments with Telnet can provide unique insights.

Of course, Telnet has its limitations too. Namely, the manual process can be slow and cumbersome compared to automated testing tools. But used judiciously alongside other methods, Telnet remains a valuable email troubleshooting tool.

When you absolutely need to get “close to the metal” on SMTP, Telnet delivers control and insights no other tool can provide. Its longevity proves the simplicity and power of the protocol.

Prerequisites for Sending Email with Telnet

Before you can start manually sending SMTP commands and test emails with Telnet, you need to set up a few prerequisites:

  • Have access to a Telnet client on your local computer
  • Find the hostname or IP address of an SMTP server
  • Verify TCP port 25 connectivity to the target SMTP server

Let’s go through each requirement for using Telnet to test email sending.

Installing a Telnet Client

Telnet functionality is included by default with most operating systems. So you likely already have access to the Telnet command.

Here’s how to check if Telnet is installed and set it up if needed:

On Windows:

  • Open the Start menu and search for “Turn Windows Features On or Off”
  • Check the box for “Telnet Client” and click OK to install it

On Linux:

  • On Ubuntu and Debian, run sudo apt install telnet
  • On Red Hat/CentOS run sudo yum install telnet

On Mac:

  • Telnet is installed by default. To enable it, go to System Preferences -> Sharing and check “Remote Login”

Alternatively, you can install a third-party Telnet utility like PuTTY or ZOC Terminal on any platform if you prefer added features and configurability.

Finding the SMTP Server’s Hostname or IP

To connect to an SMTP server with Telnet, you need to know its hostname or IP address.

If you don’t have this info handy, there are a couple ways to find it:

Check DNS MX Records

Use the nslookup -type=MX <domain> command to lookup the MX records for a domain. This will show you the hostnames of the configured SMTP servers for that domain.

For example, to find the MX records for example.com:

> nslookup -type=MX example.com 
Non-authoritative answer:
example.com mail exchanger = 10 mail.example.com

Do a Reverse DNS Lookup

If you only know the server’s IP address, do a reverse DNS lookup to find its hostname.

On Linux/Unix:

$ host domain name pointer mail.example.com

On Windows:

> nslookup
Name: mail.example.com

This allows you to connect to an SMTP server when you only have its IP.

Checking for Access on Port 25

Before trying to Telnet to an SMTP server, confirm you have network access to it on TCP port 25.

From the source server you’ll connect from, run:

$ telnet mail.example.com 25

If the connection succeeds, you will see output like:

220 mail.example.com ESMTP Service Ready

That means you can reach the SMTP server on port 25.

If it fails with “Connection refused” or timeouts, your network is likely blocking TCP/25. Work with your network admin to allow access.

You might also try connecting to TCP/465 or TCP/587, which some servers use for SMTP over SSL/TLS.

With that, you’ve validated all the prerequisites for sending email via Telnet. You have a Telnet client, the SMTP server hostname/IP, and connectivity over port 25. Time to start testing!

Step-by-Step Guide for Sending Test Emails with Telnet

Now that we’ve covered the prerequisites, let’s walk through a full example of using Telnet to connect to an SMTP server and send a test email.

We’ll cover:

  • Connecting to the SMTP port
  • Key SMTP protocol commands
  • Sending an email with all required fields
  • Understanding server response codes
  • Logging your Telnet session
  • Sending from remote servers
  • Comparing Telnet to other SMTP test tools

Follow along to get hands-on experience sending email through Telnet.

Connecting to the SMTP Server

To start, you need to connect to your target SMTP server on port 25.

Open a command prompt or terminal and run:

telnet smtp.example.com 25

This will connect to the SMTP server “smtp.example.com” on port 25.

Once connected, you will see a welcome banner like:

Connected to smtp.example.com.
Escape character is '^]'.  
220 smtp.example.com ESMTP Service Ready

The “220” response code indicates the connection was successful. You are ready to start sending SMTP commands.

SMTP Protocol Basics

SMTP has a text-based request-response structure. Your Telnet client will send SMTP commands, and the server will reply with numeric response codes and messages.

For example, once connected you need to tell the server who you are with the EHLO or HELO greeting:

EHLO myclient.example.org
250 smtp2.example.com Hello myclient.example.org

The “250” reply indicates success. You’ve now introduced yourself.

From there, you will send a set of defined commands to compose and send your test email. They are:

  • MAIL FROM: Sets the message sender
  • RCPT TO: Defines a recipient
  • DATA: Starts the email content section
  • Subject:: Specifies email subject
  • Body: Contains the email body
  • .: Ends the email content (a period on a new line)
  • QUIT: Closes the connection when done

Let’s go through each to send a test message.

SMTP Commands for Sending Email

Here are the steps to send an email over SMTP using Telnet:


As shown above, start by greeting the server:

EHLO myclient.example.org

Or with the older HELO command:

HELO myclient.example.org

This provides your domain name to the server.


Set the sender email address with MAIL FROM:

MAIL FROM: [email protected]

The server checks this is a valid address and responds with a success code.


Specify recipients with one or more RCPT TO commands:

RCPT TO: [email protected]
RCPT TO: [email protected] 

You can include multiple recipient addresses, separated by newlines.


The DATA command tells the server you are ready to provide the message content:


The server will respond with a ready code like “354”.

5. Subject

On the next lines, provide the email subject like any email client:

Subject: Test message from Telnet

6. Message Body

After a blank line, provide the body of your message:

This is the body of my test message sent from Telnet.

I can include all the standard email contents and formatting.

7. End Data

To tell the server you are done providing the message content, send a period on a new line:



Finally, close the connection cleanly with the QUIT command:


That’s it! You have manually sent a full test email via Telnet.

Understanding SMTP Response Codes

As you send SMTP commands in your Telnet session, the server will reply with 3-digit response codes that tell you how the command was handled.

Here are some common codes:

  • 220 – Server ready
  • 250 – Requested action completed
  • 354 – Continue with request (enter mail content)
  • 421 – Service shutting down and closing transmission channel
  • 500 – Syntax error, command not recognized
  • 501 – Syntax error in parameters or arguments
  • 550 – Requested action not taken (no such user, invalid address, etc.)

The first digit is most significant:

  • 2xx – Success
  • 3xx – Intermediate response
  • 4xx – Temporary error, try again later
  • 5xx – Permanent error

These codes tell you how the server handled each SMTP command you entered. They allow you to diagnose problems with mail sending.

For a full list of SMTP response codes, see the RFC 5321 list.

Sending Email to Remote Servers

If you are testing mail flow from your own Exchange or email server to another external domain, make sure to:

  • Use the local server’s real IP address in the SMTP envelope
  • Allow outbound connections to TCP/25

Use Your Server’s True IP Address

When sending from your own domain via Telnet, include your server’s public IP address in the SMTP EHLO command.

For example, from your mail server at IP sending to example.com:


This allows recipient servers to match the IP to your domain for spam/security checks.

Allow Outbound Connections on Port 25

Your local network firewall may block outbound connections to remote port 25. Work with your network team to allow SMTP access from your mail server for testing purposes.

With that setup, you can accurately test sending emails from your domain out to external recipients.

Logging Your Telnet Session

It can be useful to log your Telnet session to keep a record of the raw SMTP conversation for troubleshooting.

To enable logging in Telnet:

On Windows:

telnet> set logfile c:\telnet-log.txt

On Linux/Unix:

$ telnet 25 > smtp-log.txt

This will save the Telnet session transcript to a file. You can then review the SMTP commands and server responses during and after your test.

Telnet vs Other SMTP Test Tools

While Telnet provides the most granular control, there are some alternative tools for testing SMTP:

  • Web-based analyzers like Mail-Tester that check DNS and perform test sends.
  • Apps like Postbox that include built-in SMTP test/diagnosis functions.
  • Command-line tools like swaks designed for scripted SMTP testing.

Telnet has some advantages over these options:

More Flexible: You can interactively tweak and rerun commands, change recipients, send custom message content, etc.

Runs Anywhere: Telnet is built-in; no need to install or configure alternate tools.

Learn SMTP Internals: Manually sending commands teaches you the protocol directly.

Lightweight: Telnet has a tiny network footprint versus GUI apps or heavy testing tools.

So while not as robust as full-featured testing suites, Telnet excels when you need an easy, quick, and “close to the metal” way to verify SMTP operation and troubleshoot email sending problems.

Troubleshooting Email Sending Errors with Telnet

One of the top uses of Telnet for email troubleshooting is diagnosing SMTP errors and issues when attempting to send messages.

The direct visibility into the SMTP conversation that Telnet provides allows you to pinpoint where things are going wrong.

Let’s explore some common email sending errors and issues, and how to troubleshoot them with Telnet.

Common Telnet Connection Issues

If Telnet fails to connect to the SMTP server at all, there are a few possible causes to check:

Server Unavailable

You may see an error like:

telnet: Unable to connect to remote host: Connection refused

This indicates the SMTP service is not running or otherwise unavailable on the target server.

Verify the application status and network connectivity of your SMTP server. Restarting the service may resolve connection issues.

Firewall Blocking Access

Another connection failure possibility is your local firewall blocking access to remote port 25.

To rule this out, try connecting to port 25 on a known good server you have access to. If still failing, work with your network team to allow outbound SMTP access.

Invalid SMTP Server Address

A “could not resolve hostname” type error means the SMTP server address you provided is not valid or resolvable.

Double check you have the right server hostname or IP address. Try doing a DNS lookup to confirm the name can be resolved.

Wrong Port Number

By default SMTP uses port 25, but sometimes alternate ports are used.

If the hostname/IP is definitely correct, try connecting to a different port like 587 or 465. The banner text on successful connection will indicate the proper port.

With connection issues addressed, let’s move on to SMTP-level errors.

Fixing Invalid SMTP Command Errors

If Telnet is connecting but your SMTP commands are being rejected, the syntax of your requests is likely at fault.

For example:

250 OK  
vrfy [email protected]
500 Unrecognized command

The “500” error indicates the SMTP server did not understand your command.

Double check the spelling, capitalization, spacing, and parameters of the SMTP verbs you are sending. Consult RFC 5321 for the defined set of SMTP commands.

Troubleshooting Rejected Recipients

A common scenario is a Telnet SMTP check working fine for some recipients but failing for others with errors like:

250 Accepted  
MAIL FROM: [email protected]
250 Sender OK
RCPT TO: [email protected] 
550 No such user

Here, the recipient was rejected. This could indicate:

  • The recipient email address is invalid or non-existent. Double check it was typed correctly.
  • There is a routing issue reaching the specific recipient domain. Try other known good recipient domains.
  • Your sending IP is blacklisted or blocked for the target domain. Try sending from a different source IP.

Dealing with Authentication Issues

If you get a “relay access denied” type error when connecting, the Telnet session is likely failing the SMTP authentication check:

550 Access denied
535 Authentication failed

By default, most SMTP servers require you to authenticate using a username and password before allowing mail sending.

If relaying through your own server:

Provide valid credentials when prompted after the initial connection.

If sending to an external domain server:

You likely can’t authenticate, but some servers allow anonymous relaying. Try connecting to port 2525 instead of 25 to bypass authentication.

Some basic diligence in testing different conditions and analyzing the raw SMTP results Telnet provides will help you narrow down and resolve email sending errors.

Securing Telnet for Email Testing

While Telnet is handy for quick SMTP testing, its lack of encryption poses some security concerns. Here are tips to keep your Telnet email testing secure:

Use SSH Instead of Telnet

Telnet transmits all data including passwords in plain text. This creates risk of credentials or messages being intercepted.

A more secure alternative protocol is SSH. Enable SSH on your SMTP server and use ssh -p 25 [email protected] to connect. This will encrypt your Telnet email testing session.

However, you lose the ability to send raw SMTP commands over SSH. It simply provides an encrypted channel to the CLI. But for servers you have remote shell access to, SSH is preferred over Telnet.

Limit Accessibility to the Telnet Service

On servers where you do need to use bare Telnet, restrict access to only trusted IP addresses via firewall rules.

If possible, move the Telnet service to a non-standard, high numbered port for added obscurity.

Make sure Telnet password authentication is enabled to prevent anonymous access. Require long, complex passwords.

These measures help minimize exposure of your Telnet service to unauthorized users.

Avoid Being an Open SMTP Relay

When testing mail sending to external domains, take care not to configure your server in a way that allows unauthorized relaying of spam through it.

Some ways to prevent becoming an open relay:

  • Make sure your server requires SMTP authentication before sending messages outbound. Telnet can help test this.
  • Limit outbound SMTP connectivity from your server to only well-known major email providers like Gmail, Yahoo, and Outlook.com to prevent routing spam.
  • In inbound firewall rules, restrict SMTP connections only from known trusted partner IPs, not the whole internet.

With proper limiting of the Telnet service itself, plus good SMTP permission controls, you can safely use Telnet for SMTP testing without introducing mail relay vulnerabilities.

Alternative Approaches

If you are uncomfortable with the inherent insecurity of Telnet for email testing, some other options to consider are:

  • Web-based testing tools that use HTTPS encryption and do not require Telnet access to your server.
  • SMTP debugging built into modern mail server applications like Exchange and Postfix.
  • Capturing actual email sending traffic at the packet level using a sniffer tool like Wireshark.

Though these may require more setup and technical knowledge than ubiquitous Telnet, they avoid exposing an unencrypted protocol on your server for greater security.

At the end of the day, you have to balance functionality, convenience, and efficiency against the security risks. Used selectively in a controlled manner, Telnet can be an administrator’s best friend for quick SMTP diagnostics. But it pays to be diligent keeping your Telnet service locked down.

Key Takeaways and Next Steps

In this complete guide, you learned how to manually send email via Telnet and use it for SMTP diagnostics. Let’s review the key takeaways:

  • Telnet allows you to control the SMTP protocol directly for in-depth testing.
  • You can validate SMTP connections, send test emails, diagnose issues, and more.
  • Install a Telnet client, find your SMTP server’s address, and verify port 25 access first.
  • Walk through the SMTP commands step-by-step to send a test message.
  • Understand the different response codes returned by the SMTP server.
  • Log your Telnet session for troubleshooting transaction details.
  • Use Telnet judiciously alongside other tools like mail-tester sites.
  • Beware security issues with plain text Telnet and tighten up configurations.

With practice, you can become adept at deciphering SMTP issues using Telnet’s unique insight into email sending conversations.

Here are some suggested next steps to further master Telnet for SMTP:

Learn More SMTP Commands

Dive deeper into all the defined SMTP and ESMTP protocol commands for advanced testing. Consult RFC 5321 for the complete technical specification.

Try Scripting Telnet Sessions

Automate your Telnet testing by writing scripts to send commands and validate responses. Python, Bash, and other languages can drive Telnet programmatically.

Integrate with Monitoring Systems

Have your monitoring solution run periodic Telnet commands to check SMTP server health, and alert you of errors.

Consider Alternatives

Explore tools like swaks, mail-tester sites, Postbox’s diagnostics, or Wireshark for more robust SMTP testing capabilities.

Automate Routine Tests

Schedule a daily Telnet health-check script to run, record results, and report on any consistently failing tests.

Become a Telnet ninja, and you’ll wield an invaluable email troubleshooting skill that never goes out of style. Telnet offers a powerfully direct view into the SMTP protocol that no other tool provides.

Key Takeaways for Sending Email via Telnet

Here are the major takeaways from this comprehensive guide on using Telnet to send email and test SMTP:

  • Telnet lets you interact directly with SMTP servers using raw protocol commands. This is useful for diagnosing email issues.
  • Install a Telnet client on your computer, find the SMTP server’s address, and confirm TCP port 25 connectivity first.
  • Walk through the steps of opening a connection, issuing SMTP commands like EHLO, MAIL FROM, RCPT TO, and DATA to send a test message.
  • Understand common SMTP response codes like 220, 250, 554, etc. to interpret server replies.
  • Log your Telnet session into a file for later review when troubleshooting problems.
  • Scripting Telnet can help automate SMTP testing. But manual interactive use is best for learning the protocol.
  • Combine Telnet with web-based testers, packet sniffing, and other tools for comprehensive diagnostics.
  • Secure your Telnet installation and connections to avoid exposing risks like open relays or unencrypted passwords.
  • Telnet offers direct low-level SMTP visibility that complements higher-level email sending tools and monitoring.

With practice, Telnet can become an invaluable mail flow troubleshooting tool for any email administrator by revealing the underlying SMTP conversation details.

Frequently Asked Questions About Sending Email with Telnet

Q: What Telnet client should I use to test SMTP?

A: The built-in Telnet client included with Windows, Linux, or Mac OS is fine in most cases. For added functionality, you can try third party clients like PuTTY, ZOC, or Netcat. But the basic Telnet command line tool is all you really need.

Q: Where do I find the address of my mail server for Telnet?

A: Check your MX records by doing nslookup -type=MX domainname.com. Or if you only have an IP, do a reverse lookup to find the hostname. Also try scanning common SMTP ports 25, 465, 587 on your mail server IPs.

Q: Why am I seeing “Connection refused” errors in Telnet?

A: A connection refusal means your Telnet client can’t reach the destination SMTP server at all. Possible causes are incorrect IP/hostname, firewall blocking, unavailable SMTP service, or incorrect port.

Q: Do I need to authenticate when using Telnet to send email?

A: Most SMTP servers require authentication before allowing mail sending. Provide credentials after connecting when prompted, or connect anonymously if the server allows it. You may need to enable anonymous relaying for testing purposes.

Q: How can I run automated SMTP tests with Telnet?

A: Script your Telnet sessions and commands using Python, Bash, PowerShell, or other languages. Validate expected responses programmatically. Schedule automated tests for regular health checks.

Q: Is there an easier way to test SMTP than Telnet?

A: Web-based testers like mail-tester.com provide a GUI and don’t require command line skills. Built-in diagnostics in email clients and servers can also simplify testing. But Telnet offers the most detailed low-level control.

Q: What are some key SMTP response codes I may see in Telnet?

A: 220 (Ready), 250 (OK), 354 (Start mail input), 421 (Closing connection), 500 (Syntax error), 550 (Not accepted), and 554 (Transaction failed) are some of the most common SMTP responses.