The receiving server did not recognize the SMTP command sent by your mail server. This usually indicates a protocol compatibility issue, misconfigured sending server, or a firewall/security appliance interfering with the SMTP conversation.
What Does Error 500 Mean?
SMTP code 500 means the receiving server received an SMTP command it does not recognize. This is a protocol-level error that usually indicates a misconfiguration on the sending side or network interference. Common causes include firewalls or anti-virus appliances that modify SMTP traffic, TLS negotiation failures that corrupt the data stream, or sending software that uses non-standard SMTP extensions.
In modern email infrastructure, 500 errors are relatively rare because most MTAs comply with standard SMTP commands. If you encounter this error, it often points to a network appliance (firewall, proxy, or anti-virus gateway) that is intercepting and modifying the SMTP conversation. Some older mail servers may also return 500 for commands they do not support, like STARTTLS.
Common Causes
- Firewall or security appliance modifying SMTP traffic
- TLS negotiation failure corrupting the command stream
- Sending server using non-standard SMTP extensions
- Line ending issues (CR/LF formatting) in SMTP commands
- Proxy or load balancer interfering with the SMTP protocol
How to Fix Error 500
- Check for firewalls or security appliances that inspect SMTP traffic
- Verify TLS configuration on your sending server
- Test sending with a basic SMTP client (telnet or openssl s_client) to isolate the issue
- Review your MTA configuration for non-standard commands or extensions
- Ensure proper CR/LF line endings in SMTP commands
Frequently Asked Questions
SMTP error 500 means "Syntax error, command unrecognized," indicating a communication failure between the sending and receiving mail servers. The receiving server could not understand or does not support the SMTP command that was sent to it. This permanent error is typically caused by a malformed command, a firewall or antivirus program interfering with SMTP traffic, an incompatibility between the servers' supported SMTP protocol versions, or the SMTP command line exceeding the maximum allowed length.
To fix a 500 syntax error, first check if a firewall, antivirus, or email security appliance is inspecting and modifying SMTP traffic -- temporarily disable these to test if they are the cause. Verify that your email client or application is using properly formatted SMTP commands and that the command line length does not exceed server limits. Check your server logs for detailed error messages pointing to the specific command that failed. If the error involves STARTTLS, ensure both your client and the server support TLS and that the correct port (587 for STARTTLS) is being used.
Yes, firewalls are one of the most common causes of SMTP error 500. Many firewalls and security appliances perform deep packet inspection on SMTP traffic, which can corrupt, modify, or strip SMTP commands before they reach the mail server. When the server receives a garbled or incomplete command, it responds with a 500 syntax error. To diagnose this, temporarily bypass the firewall for SMTP traffic, check firewall logs for SMTP-related events, and ensure the firewall's SMTP proxy or ALG (Application Layer Gateway) is not interfering with mail delivery.
"500 command not recognized" means the receiving mail server received an SMTP command it does not understand or support. This can happen when the sending server uses an extended SMTP (ESMTP) command like STARTTLS or AUTH that the receiving server has not implemented, when a firewall strips or rewrites parts of the command, or when there is a software bug or misconfiguration in the sending application. Check which SMTP command triggered the error in your server logs and verify the receiving server supports it by reviewing its EHLO response capabilities.
SMTP error 500 is a permanent failure (hard bounce) because it is a 5xx-class response code. The server will not automatically retry delivery because the same command will produce the same error. However, unlike recipient-related permanent errors (like 550 for an invalid address), a 500 error is usually a technical configuration issue that can be resolved. Once you fix the underlying cause -- such as correcting a firewall rule, updating SMTP client software, or adjusting command formatting -- the email can be sent successfully.