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SMTP Error 555: Parameters Not Recognized or Not Implemented

SMTP error 555 indicates that MAIL FROM or RCPT TO parameters are not recognized or not implemented. This error occurs when you use parameters in MAIL FROM or RCPT TO commands that the server doesn't recognize or support.

What Does 555 Mean?

The SMTP reply code 555 follows the standard SMTP response format:

  • 5 = Permanent failure (permanent negative completion reply)
  • 55 = MAIL FROM/RCPT TO parameters not recognized or not implemented

When you receive a 555 error, it means the mail server doesn't recognize or support the parameters you used in the MAIL FROM or RCPT TO commands. This is a permanent error that requires removing or changing the parameters.

Error Type

  • Type: Permanent failure (hard error)
  • Category: Parameter support
  • Action Required: Remove or change the unsupported parameters

Common Causes

  1. Unsupported Parameter: The parameter is not supported by the server
  2. Parameter Format: The parameter format is incorrect
  3. Feature Not Enabled: The feature associated with the parameter is not enabled
  4. Server Limitations: The server has limitations that prevent parameter support
  5. Protocol Version: Using parameters from a newer protocol version
  6. Server Configuration: Server configuration doesn't allow 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. Remove Parameters: Try the command without the unsupported parameters
  3. Use Alternative Parameters: Use alternative parameters that achieve the same result
  4. Update Server: If possible, update the server to support the parameters
  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 555 errors gracefully by removing unsupported parameters
  5. Parameter Logging: Log 555 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

555 MAIL FROM/RCPT TO parameters not recognized or not implemented

Example with Enhanced Status Code

555 5.5.4 MAIL FROM parameter not recognized

Common Scenarios

  • DSN Parameter: "555 DSN parameter not recognized"
  • SIZE Parameter: "555 SIZE parameter not implemented"
  • AUTH Parameter: "555 AUTH 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 555 errors gracefully by removing unsupported parameters
  5. Log for Analysis: Log 555 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