SMTP Error 500 5.5.1 – How to Fix It [SOLVED]

SMTP Error 500 5.5.1 is a permanent delivery failure caused by a syntax error where the receiving server does not recognize the sender’s command or parameters. To resolve this, senders should:

• Ensure you have the latest version of your email client.
• Review and confirm if SMTP settings and SMTP command sequence are properly set up.
• Check for syntax errors within your code if you are using automated scripts. Make sure all SMTP commands and email addresses adhere to strict formatting rules.
• Temporarily disable any antivirus or firewall software to confirm if it is intercepting SMTP commands.

There are many harmful SMTP errors, but SMTP Error 500 5.5.1 – Syntax error, Command Unrecognized can be more damaging than others if not acted upon right away.

This guide will walk you through everything you need to know about SMTP Error 500 5.5.1 and how to prevent it from disrupting your email campaigns and damaging your sender reputation.

SMTP Error 500 5.5.1 syntax error: What is it and what does it mean?

Every SMTP error that belongs in the 500 code family indicates a permanent syntax error and the enhanced status code 5.5.1 provides more details.

SMTP Error 500 5.5.1 means that the receiving server is rejecting an unrecognized command or malformed parameters. It is a permanent delivery failure, which means it cannot be resolved on its own. It doesn’t automatically retry, and manually resending the email won’t solve it either.

Error messages that accompany the error code can be any of the following:

• Syntax error, Command Unrecognized
• Authentication Needed
• Syntax error in parameters or arguments

This is a different story from other authentication errors wherein the server understands you but won’t let you through, or mailbox errors where the command is correct but the destination doesn’t exist. 

With Error 500 5.5.1, the server is essentially saying, “I don’t know what you’re asking me to do.”

How to fix SMTP Error 500 5.5.1

Understanding why this error occurs helps you troubleshoot more effectively. Here are the most frequent culprits behind this error and the steps you can take to resolve them.

1. Make sure all SMTP commands are valid

When your sending client or server issues commands the receiving server doesn’t recognize or support, it triggers SMTP Error 500 5.5.1. 

Here’s what that looks like:

• SMTP relies on specific commands like HELO, EHLO, MAIL FROM, RCPT TO, AUTH, and STARTTLS. 
• If any of these contain typos, incorrect spacing (e.g., missing CRLF), malformed arguments (e.g., unbracketed addresses), or unsupported extensions/protocol versions, the receiving server will reject them

How to fix this: 

• Make sure that you have the latest email client version. If you don’t, then make sure you update them. Many developers employ regular patch bugs that cause malformed SMTP commands. 
• Review your SMTP command syntax especially if you are using a custom email script or application. 

2. Ensure command sequence is correct 

Each command sequence must follow its specific order. Incorrect command sequence creates a miscommunication between two email servers, since the server expects a conversation to follow a particular flow. So when the sequence breaks, the server is left clueless, resulting in “command unrecognized.”

This issue commonly occurs with custom email scripts or when developers are building their own email sending functionality without fully understanding the SMTP protocol handshake.

How to fix this: 

• Review the full error message that accompanies the 500 5.5.1 code. It often provides more detail on which command failed, such as “Command unrecognized,” “Authentication Required,” or “EHLO/HELO first.” This information points you directly to what went wrong in the sequence.
• Once you’ve figured it out, you need to make sure that your code follows the appropriate SMTP command sequence:

• • Connection, greeting (EHLO/HELO)
  • Authentication (if required)
  • Sender specification (MAIL FROM)
  • Recipient specification (RCPT TO)
  • Data transmission
  • Disconnection.

3. Fix authentication issues

This isn’t about a simple password or credential mismatch. Authentication issues in SMTP appear when there is an issue with the authentication command, such as:

• Your email client isn’t configured to authenticate
• It does not have the correct format and is then unrecognizable
• It is attempting to use an authentication method the server doesn’t support

How to fix this: 

• Confirm if your SMTP settings and server address is correct. If you are using Gmail then make sure that it is set in: smtp.gmail.com. If you are using Outlook then then it should be smtp-mail.outlook.com
• Then use the recommended port number: port 587 with TLS/STARTTLS or port 465 with SSL. 
• Ensure that “My outgoing server requires authentication” is enabled in your email client settings, and confirm your username and password are correct. 

Ensuring your authentication protocols are properly configured, as this helps prevent permanent rejections like SMTP Error 5.3.3. You can check this through our free email deliverability test.

4. Identify and resolve malformed syntax in email headers

If you’ve already verified that you have the correct SMTP commands, you need to start checking the content those commands are trying to send.

Sometimes this error occurs due to improperly formatted email headers that causes the server to reject the entire command. For example, missing angle brackets around email addresses in MAIL FROM or RCPT TO commands, or improperly structured headers, can trigger this error.

How to fix this: 

• If you’re a developer using automated scripts, check for syntax errors in your code. 
• Make sure all of your SMTP commands and email address follow strict formatting rules. It is important to note that email addresses in SMTP commands should be enclosed in angle brackets.

5. Make sure there are no antivirus and firewall interference

Security software, firewalls, or proxy servers sitting between your email client and the destination server can modify or corrupt SMTP commands in transit. 

Deep packet inspection, certain antivirus scanning features, or poorly configured proxy servers might alter command syntax just enough to make them unrecognizable to the receiving server.

How to fix this: 

• Temporarily disable any antivirus or firewall software and confirm if it is intercepting or change SMTP commands.
• If disabling security software resolves the issue then that’s it. 
• However, if that isn’t the case then you might need to work with your IT department (if you are using an email from a corporation) to guarantee that security protocols are not modifying the SMTP traffic in ways that break command syntax. Some modern antivirus programs have this option to disable email scanning, while it maintains other security measures, causing this error to occur. 

6. When nothing works, contact your provider

Contact your email service provider’s support team for assistance. They have access to server-side logs that can reveal exactly which command is failing and why. They can provide specific guidance for their system and identify whether there are known issues or special configuration requirements you need to follow. Sometimes the issue involves server-side settings that only the provider can adjust.

Will your email deliverability be affected by SMTP Error 500 5.5.1?

Yes and no. 

Here’s what we mean by this: In the short term, no. Experiencing it once will not automatically damage your deliverability. But if you experience it often, it may be the symptom of a bigger problem.

There is a long list of SMTP errors and codes that you may experience. And like other errors in the 5** variants,  SMTP Error 500 5.5.1 represents a permanent failure, where simply waiting out and resending would not work. 

For individual users sending occasional emails, this error is more of an inconvenience than a crisis. You’ll know immediately that something went wrong and can troubleshoot accordingly.

But how about businesses? If you are managing multiple domains and are experiencing this error repeatedly, then the error can be systemic and may suggest a problem at your email infrastructure. 

Repeated permanent failures impact your sender reputation with email service providers. When your domain or IP address consistently generates hard bounces, ISPs and email platforms take notice. They may interpret this pattern as a sign of poor list hygiene, technical incompetence, or even potential malicious activity

Over time, this will degrade your sender score, which can result in legitimate emails being filtered to spam or blocked entirely, even after you’ve resolved the original 500 5.5.1 error.

How Warmy protects your email infrastructure from SMTP errors

While understanding and fixing SMTP Error 500 5.5.1 is important, preventing email deliverability problems before they occur is even better. 

This is where Warmy.io becomes an invaluable part of your email infrastructure.

Warmy is an email warmup and deliverability platform designed to help businesses establish and maintain healthy sender reputations and maximize inbox placement rates. Rather than waiting for errors to appear and then scrambling to fix them when they do, Warmy helps users establish a high deliverability rate. This prevents many SMTP errors from occurring in the first place.

Warmy helps build sender reputation that withstands technical hiccups