SMTP Error 502 5.5.1 – Command Not Implemented: How to Fix It

What is SMTP Error 502 5.5.1?

SMTP Error 502 5.5.1 is an error message returned from the mail server of the mail client, informing the mail client that the mail server does not recognize or support an SMTP command. Now, let’s dissect the error code:

• 502: Standard SMTP status code. You can explore other SMTP Errors with 502 code here.
  
• 5.5.1; Enhanced status code indicating a syntax or command issue
  

This typically happens during the initial handshake between the email client and the SMTP server. For example, your client might send commands like:

• EHLO: used to identify the client and request server capabilities
• AUTH: used to start the authentication process
• STARTTLS: used to initiate a secure connection
  

When and where you might encounter it

Generally speaking, SMTP Error 502 5.5.1 appears in cases where your email client or application is attempting to talk to an SMTP server that does not support the command you’re trying to send. Common situations include:

• Sending email via an SMTP relay. If you’re sending messages via a third-party SMTP relay (like SendGrid, Amazon SES, or even your hosting provider), the relay server may not understand, refusing unsupported or malformed commands.
  
• Using third-party email tools or custom apps. Automation tools, scripts, or integrations (like Python scripts, CRMs, or internal email bots) can trigger this error if they use outdated or incompatible SMTP libraries.
  
• Specific clients: Outlook, Thunderbird, or custom scripts. Some email clients may default to sending certain SMTP commands that aren’t accepted by the server—especially when misconfigured or outdated.
  

Common causes of SMTP Error 502 5.5.1

The most common cause of SMTP Error 502 5.5.1 is a compatibility or configuration problem between the email client and the SMTP server. Here are the most common reasons this error happens:

Unsupported SMTP commands

SMTP commands like EHLO, AUTH, or STARTTLS are used during the handshake process, and if the server doesn’t support one of them—or expects a different syntax—it will return the “Command not implemented” response. This usually points to a lack of compatibility between the client and server.

Q: What is the handshake process?

A: The SMTP handshake is the first step of the interaction between the email client (the sender) and an SMTP server (the recipient). All of this happens when the client sends a HELO or EHLO command to identify itself, and ask for the capabilities of the server. It also gives its state and what commands are available, including authentication methods and encryption supported by the server. This handshake makes sure that both the client and server can communicate successfully before the actual email transmission starts.

Outdated or misconfigured email clients

Email clients that use outdated libraries or are misconfigured may rely on deprecated SMTP protocols. When these clients attempt to connect using old standards or incorrect authentication methods, the server may reject the request with a 502 error. Common examples include legacy software or older versions of email clients that haven’t been updated.

SMTP server limitations

SMTP servers may block or disable certain features for various reasons, including policy restrictions, security concerns, or lack of server resources. Specifically, a server may refuse encrypted connections over STARTTLS or may disable authentication on various ports. If this is the case, even valid commands may be rejected if the server is not configured to handle them.

Authentication mismatch or issues

If your email client attempts to use an authenticate method that the server doesn’t support—such as XOAUTH2, plain text login without encryption, or a mismatched encryption type (SSL vs TLS)—this could result in a 502 5.5.1 error response from the server. This usually occurs when client configurations do not match authentication schemes supported by the server.

Q: What is SSL and TLS?

A: SSL (Secure Sockets Layer) and TLS (Transport Layer Security) are protocols of applied cryptography for securing communication over a computer network. TLS is the newer and more secure version of SSL, providing encryption over the data sent/received by the client and the server.

🔖 Related Reading: Why SSL & TLS Certificate Errors Are Hurting Your Email Deliverability

Incorrect SMTP server configuration

Last but not least, internal server misconfigurations including incorrect port usages, blocking SMTP traffic via the firewalls and proxy servers that block email communication may also result in command failures. Such types of issues usually cause the SMTP handshake to fail, returning the 502 5.5.1 error response.

How to diagnose the problem

To diagnose the SMTP Error 502 5.5.1, you must review a few critical areas to determine what the issue is. Use these steps to help narrow down the source of the problem. 

Step 1. Review the full error message

Start by looking at the full error message returned by the server. It may provide clues about which specific command is causing the issue. You might see something like:

• Command not implemented
• Unknown command

Look for any command names (like EHLO, AUTH, STARTTLS) or error codes that can indicate which part of the connection process failed. This first glance may provide you with a clearer picture of what’s going haywire, and where you can focus your troubleshooting efforts.

Step 2. Check SMTP command logs

• Enable verbose logging. Many email clients and SMTP libraries offer verbose logging. Enable this feature to capture all interaction details.
  
  
• Tools to use:
  
  • Telnet: You can use Telnet to manually connect to the SMTP server and send test commands.
    
  • Curl: Curl can also be used to interact with the SMTP server and observe responses.
    
  • Swaks: This tool allows for detailed testing of SMTP communication.
    

These tools will help you see the actual commands being sent and identify any discrepancies or issues with the server’s responses.

Step 3. Compare client and server SMTP settings

• Look for mismatches. Check for differences in things like:
  
  • Protocol versions (e.g., TLS vs SSL)
  • Supported commands (EHLO, AUTH, STARTTLS)
    Authentication methods and ports
    
• Ensure consistency. Verify compatibility between client and server (including protocol versions)

A mismatch between the client and server settings is a common cause of this error, and identifying them will often resolve the issue. 

Fixes and workarounds for SMTP Error 502 5.5.1

Fix 1. Update your email client or library

• Be sure you are using the most up to date version of your email client or the email library within your code.
  
• For email clients: Update programs like Thunderbird, Outlook, or whatever email software you’re using. They may have fixed compatibility issues with certain SMTP commands in newer versions.
  
• For code libraries: If you’re using a library like Python’s smtplib, ensure it’s up to date with the latest standards and commands.
  

Fix 2. Use supported commands only

• Verify commands. Make sure the commands EHLO, AUTH, and STARTTLS are supported by your server. If these aren’t supported, the server can respond with a 502 error. 
  
• Downgrade to HELO if necessary. If EHLO isn’t working, try switching to HELO. However, use caution, as downgrading can limit the server’s capabilities (e.g., no TLS support).
  
  

Fix 3. Configure authentication correctly

• Use the correct authentication method (usually plain authentication or login). If you’re unsure, refer to your server’s documentation.
  
• Avoid XOAUTH. If your server doesn’t support XOAUTH (a modern OAuth method), do not try to use it. Instead, stick to simpler methods like username/password authentication.
  

Fix 4. Switch to another port or protocol

In case you are facing issues with the default SMTP port (25), you can switch to another port or protocol. These alternate ports can often bypass server restrictions or firewall blocks. 

• Port 587: Try using port 587 with TLS for a secure connection. This is the recommended port for SMTP submission.
  
• Port 465: If port 587 isn’t working, try using port 465, which supports SMTPS (SMTP over SSL).
  

Fix 5. Contact your email service provider

If the above fixes don’t work and you’re still facing the error, it’s time to contact your email service provider. Your provider can give you the most accurate information and may even offer a server-side fix.

• Request supported commands. Ask your provider for a list of SMTP commands that their server supports.
  
• Check if 502 5.5.1 is a known issue. Some email providers might already be aware of this issue and could have a specific workaround or patch in place.
  

How Warmy.io can help preventing SMTP Error 502 in the future

Prevention is the vital step in improving your email deliverability, but one must keep in mind that fixing this particular error is not the guaranteed solution for reaching the inbox 100% of the time. There are many elements that impact the destination of your emails—like sender reputation, content quality, and server-side configurations.

However, this is where Warmy.io comes in. Warmy offers a suite of features designed to significantly optimize your email flow, minimize deliverability issues, and ensure your emails have the best chance of making it to the inbox—despite the many factors at play.

AI-powered email warmup to build and maintain sender reputation

Warmy.io’s email warmup feature gradually builds trust with ISPs, ensuring that your emails are recognized as legitimate. It mimics natural email interactions to show ISPs that your emails and your domain are trusted and valuable. This tool is capable of sending 5,000 emails in one single day. It also offers custom templates and multiple languages for customization on the recipient side.