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SMTP Bounce Code 5.1.3: Bad Address Syntax

SMTP bounce code 5.1.3 indicates that the recipient's email address has invalid syntax and doesn't conform to the standard email address format. This is a permanent failure (hard bounce), meaning the address format is fundamentally incorrect and cannot be delivered.

What Does 5.1.3 Mean?

The enhanced status code 5.1.3 follows the SMTP Enhanced Status Code format:

  • 5 = Permanent failure (hard bounce)
  • 1 = Addressing status (related to the recipient address)
  • 3 = Bad destination mailbox address syntax

When you receive a 5.1.3 bounce, it means the email address format is syntactically invalid according to RFC standards. The address cannot be parsed or processed because it doesn't follow the proper format (localpart@domain).

Bounce Type

  • Type: Hard bounce (permanent failure)
  • Category: Addressing
  • Action Required: Remove the email address from your mailing list or correct the syntax

Common Causes

  1. Missing @ Symbol: The email address is missing the @ symbol (e.g., userexample.com)
  2. Invalid Characters: The address contains invalid characters that aren't allowed in email addresses
  3. Multiple @ Symbols: The address contains multiple @ symbols (e.g., user@@example.com)
  4. Spaces in Address: The address contains spaces (e.g., user [email protected])
  5. Missing Local Part: The address is missing the local part before @ (e.g., @example.com)
  6. Missing Domain: The address is missing the domain part after @ (e.g., user@)
  7. Invalid Domain Format: The domain portion has invalid format (e.g., [email protected], user@example)
  8. Special Character Issues: Improper use of special characters (quotes, backslashes, etc.)
  9. Consecutive Dots: The address contains consecutive dots (e.g., [email protected])
  10. Leading/Trailing Dots: The local part starts or ends with a dot (e.g., [email protected])

How to Resolve

For Email Marketers

  1. Remove from List: Immediately remove the email address from your active mailing list
  2. Check for Typos: Review the address for obvious typos or formatting errors
  3. Validate Address Format: Use email validation tools to check address syntax
  4. Correct if Possible: If you can identify the correct format, update the address (e.g., fix user@@example.com to [email protected])
  5. Review Collection Methods: Review how you collected this email address to prevent similar issues
  6. Use Validation on Input: Implement email format validation when collecting addresses

For Developers

  1. Email Format Validation: Implement strict email format validation using RFC 5322 compliant regex
  2. Syntax Checking: Validate email syntax before storing addresses in your database
  3. Implement Bounce Handling: Set up automated bounce processing to remove 5.1.3 bounces
  4. Input Sanitization: Sanitize and validate email addresses at the point of entry
  5. Pattern Analysis: Analyze 5.1.3 bounces to identify common syntax errors and improve validation
  6. Address Normalization: Normalize email addresses (trim whitespace, convert to lowercase) before validation
  7. Prevent Duplicate @: Check for multiple @ symbols before sending
  8. Suppression List: Maintain a suppression list of addresses with syntax errors

Email Address Format Rules

A valid email address must:

  • Contain exactly one @ symbol
  • Have a local part (before @) that is not empty
  • Have a domain part (after @) that is not empty
  • Follow RFC 5322 format specifications
  • Not contain spaces (unless properly quoted)
  • Not start or end with a dot in the local part
  • Not contain consecutive dots

Related Bounce Codes

Examples

Example Bounce Message

550 5.1.3 Bad address syntax
Email address format is invalid.

Example Enhanced Status Code

550 5.1.3 <user@@example.com>: Bad address syntax - multiple @ symbols

Common Email Provider Responses

  • Generic: "550 5.1.3 Bad address syntax"
  • Multiple @: "Invalid address format - multiple @ symbols"
  • Missing @: "Email address must contain @ symbol"
  • Invalid Format: "Email address format is invalid"

Common Syntax Errors

Best Practices

  1. Never Retry: Don't attempt to resend emails to addresses with syntax errors—they will continue to fail
  2. Validate on Input: Always validate email address format when users enter their email
  3. Use RFC-Compliant Validation: Use RFC 5322 compliant validation rather than simple regex patterns
  4. Normalize Addresses: Normalize email addresses (trim, lowercase) before validation
  5. Prevent Common Errors: Check for common errors like multiple @ symbols, spaces, etc.
  6. Maintain List Hygiene: Regularly clean your email list to remove addresses with syntax errors
  7. Use Email Validation Libraries: Use established email validation libraries rather than custom regex
  8. Log Syntax Errors: Log syntax errors to identify patterns and improve validation