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SMTP Error 504: Command Parameter Not Implemented

SMTP error 504 indicates that a command parameter is not implemented on the mail server. This error occurs when you send a valid command with a parameter that the server doesn't support.

What Does 504 Mean?

The SMTP reply code 504 follows the standard SMTP response format:

  • 5 = Permanent failure (permanent negative completion reply)
  • 04 = Command parameter not implemented

When you receive a 504 error, it means the mail server recognizes the command but doesn't support the specific parameter you provided. This is a permanent error that requires using a different parameter or command.

Error Type

  • Type: Permanent failure (hard error)
  • Category: Parameter support
  • Action Required: Use a different parameter or command

Common Causes

  1. Unsupported Parameter: The parameter is not supported by the server
  2. Feature Not Enabled: The feature associated with the parameter is not enabled
  3. Server Limitations: The server has limitations that prevent parameter support
  4. Protocol Version: Using parameters from a newer protocol version
  5. Server Configuration: Server configuration doesn't allow the parameter
  6. Security Restrictions: Security policies prevent the parameter
  7. Legacy Server: Older server versions don't support newer parameters

How to Resolve

For Email Senders

  1. Check Server Capabilities: Verify which parameters the server supports using EHLO
  2. Use Alternative Parameters: Use alternative parameters that achieve the same result
  3. Remove Parameter: Try the command without the unsupported parameter
  4. Update Server: If possible, update the server to support the parameter
  5. Contact Administrator: Contact the server administrator to enable the feature

For Developers

  1. Check Server Capabilities: Always check server capabilities using EHLO before using parameters
  2. Implement Fallbacks: Implement fallback mechanisms for unsupported parameters
  3. Feature Detection: Detect server capabilities and adjust parameters accordingly
  4. Error Handling: Handle 504 errors gracefully with appropriate fallbacks
  5. Parameter Logging: Log 504 errors to track unsupported parameters
  6. Use SMTP Libraries: Use well-tested SMTP libraries that handle parameter support

Related SMTP Errors

Examples

Example Error Message

504 Command parameter not implemented

Example with Enhanced Status Code

504 5.5.4 Command parameter not implemented

Common Scenarios

  • AUTH Mechanism: "504 AUTH mechanism not supported"
  • SIZE Parameter: "504 SIZE parameter not implemented"
  • DSN Parameter: "504 DSN parameter not supported"

Best Practices

  1. Check Capabilities First: Always check server capabilities using EHLO
  2. Implement Fallbacks: Have fallback mechanisms for unsupported parameters
  3. Feature Detection: Detect and adapt to server parameter support
  4. Handle Gracefully: Handle 504 errors gracefully without breaking functionality
  5. Log for Analysis: Log 504 errors to identify commonly unsupported parameters
  6. Document Requirements: Document which parameters are required vs optional
  7. Test Compatibility: Test against multiple SMTP servers with different parameter support
  8. Update Servers: Keep servers updated to support modern SMTP parameters