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SMTP Error 501: Syntax Error in Parameters or Arguments

SMTP error 501 indicates that a syntax error occurred in the parameters or arguments of a command. This error occurs when the command itself is recognized, but the parameters provided are invalid or malformed.

What Does 501 Mean?

The SMTP reply code 501 follows the standard SMTP response format:

  • 5 = Permanent failure (permanent negative completion reply)
  • 01 = Syntax error in parameters or arguments

When you receive a 501 error, it means the mail server recognized the command but cannot parse or validate the parameters you provided. This is typically a permanent error that requires fixing the parameter syntax.

Error Type

  • Type: Permanent failure (hard error)
  • Category: Parameter syntax
  • Action Required: Fix the parameter syntax

Common Causes

  1. Invalid Email Address: The email address format is incorrect
  2. Missing Parameters: Required parameters are missing from the command
  3. Invalid Characters: Parameters contain invalid characters
  4. Wrong Parameter Format: Parameters are in the wrong format
  5. Encoding Issues: Character encoding problems in parameters
  6. Special Characters: Unescaped special characters in parameters
  7. Parameter Length: Parameters exceed maximum length limits
  8. Domain Format: Invalid domain name format

How to Resolve

For Email Senders

  1. Validate Email Addresses: Ensure email addresses are properly formatted
  2. Check Parameters: Verify all required parameters are provided
  3. Review Command Syntax: Review the SMTP command syntax for the command you're using
  4. Test with Simple Address: Try with a simple email address to isolate the issue
  5. Update Client Software: Update your email client or SMTP library

For Developers

  1. Validate Parameters: Validate all parameters before sending SMTP commands
  2. Email Validation: Use proper email address validation
  3. Parameter Encoding: Properly encode parameters according to SMTP standards
  4. Error Handling: Implement proper error handling for 501 errors
  5. Parameter Logging: Log parameters that result in 501 errors for debugging
  6. Use SMTP Libraries: Use well-tested SMTP libraries that handle parameter validation

Related SMTP Errors

Examples

Example Error Message

501 Syntax error in parameters or arguments

Example with Enhanced Status Code

501 5.5.4 Syntax error in parameters

Common Scenarios

  • Invalid MAIL FROM: "501 5.5.4 Invalid address"
  • Invalid RCPT TO: "501 5.5.4 Invalid recipient address"
  • Invalid HELO Domain: "501 5.5.4 Invalid domain name"

Best Practices

  1. Validate Before Sending: Always validate parameters before sending SMTP commands
  2. Use Email Validation: Use proper email address validation libraries
  3. Handle Errors: Implement proper error handling for 501 errors
  4. Log Parameters: Log parameters that cause 501 errors for debugging
  5. Follow Standards: Ensure parameter format follows SMTP standards
  6. Test Edge Cases: Test with various email address formats
  7. Update Libraries: Keep SMTP libraries updated
  8. Sanitize Input: Sanitize user input before using in SMTP commands