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SMTP Bounce Code 5.5.0: Other Protocol Status

SMTP bounce code 5.5.0 indicates that there is a problem with the SMTP protocol or mail delivery protocol that doesn't fit into a more specific category. This is a permanent failure (hard bounce), meaning there's a fundamental protocol issue that prevents message delivery.

What Does 5.5.0 Mean?

The enhanced status code 5.5.0 follows the SMTP Enhanced Status Code format:

  • 5 = Permanent failure (hard bounce)
  • 5 = Mail delivery protocol status
  • 0 = Other or undefined protocol status

When you receive a 5.5.0 bounce, it means the receiving mail server has identified a protocol problem that doesn't fall into more specific categories. This is a catch-all code for various SMTP protocol issues.

Bounce Type

  • Type: Hard bounce (permanent failure)
  • Category: Protocol
  • Action Required: Remove the email address from your mailing list or investigate the protocol issue

Common Causes

  1. Protocol Version Mismatch: Incompatibility between SMTP protocol versions
  2. Command Syntax Errors: SMTP command syntax issues that don't fit other categories
  3. Protocol Extension Issues: Problems with SMTP extensions or capabilities
  4. Authentication Protocol Problems: Issues with authentication protocol implementations
  5. Protocol State Errors: Errors in SMTP protocol state machine
  6. Command Sequence Issues: Problems with SMTP command sequences
  7. Protocol Feature Incompatibility: Protocol features that are incompatible between servers
  8. TLS/SSL Protocol Issues: Problems with TLS/SSL protocol implementation
  9. Protocol Configuration Errors: Server protocol configuration issues
  10. Generic Protocol Error: A protocol error that doesn't fit into more specific categories

How to Resolve

For Email Marketers

  1. Remove from List: Remove the email address from your active mailing list immediately
  2. Contact Recipient: If possible, contact the recipient through alternative channels to verify their mail server status
  3. Document the Issue: Document which domains are affected by protocol errors
  4. Monitor for Recovery: Periodically check if the issue has been resolved
  5. Use Alternative Channels: For important communications, use alternative contact methods

For Developers

  1. Remove from Database: Immediately remove the email address from your sending database
  2. Implement Bounce Handling: Set up automated bounce processing to remove 5.5.0 bounces from your database
  3. Log Protocol Errors: Log the full bounce message and SMTP conversation to understand the protocol issue
  4. Review SMTP Implementation: Review your SMTP client implementation for protocol compliance
  5. Test Protocol Compatibility: Test your SMTP implementation against various mail servers
  6. Suppression List: Maintain a suppression list of addresses with protocol errors
  7. Protocol Debugging: Enable SMTP protocol debugging to identify issues

Investigation Steps

When you receive 5.5.0 bounces, investigate:

  1. Check Full Bounce Message: Review the complete bounce message for specific protocol details
  2. Review SMTP Logs: Check SMTP conversation logs to identify protocol issues
  3. Test Protocol Compatibility: Test SMTP protocol compatibility with the receiving server
  4. Verify SMTP Implementation: Ensure your SMTP client follows RFC standards
  5. Contact Support: Contact the recipient's IT department for more information

Related Bounce Codes

Examples

Example Bounce Message

550 5.5.0 Other protocol status
SMTP protocol error occurred during message transmission.

Example Enhanced Status Code

550 5.5.0 <[email protected]>: Other protocol status - protocol version mismatch

Common Email Provider Responses

  • Generic: "550 5.5.0 Other protocol status"
  • Protocol Error: "SMTP protocol error"
  • Version Mismatch: "Protocol version incompatible"
  • Command Error: "SMTP command sequence error"

Best Practices

  1. Never Retry: Don't attempt to resend emails to addresses that bounce with 5.5.0—they will continue to fail
  2. Use RFC-Compliant SMTP: Ensure your SMTP implementation follows RFC standards (RFC 5321, RFC 5322)
  3. Log Protocol Details: Keep detailed logs of SMTP conversations to identify protocol issues
  4. Test Compatibility: Test your SMTP implementation against various mail servers
  5. Maintain List Hygiene: Regularly clean your email list to remove addresses with protocol errors
  6. Use Suppression Lists: Maintain suppression lists to prevent sending to addresses with protocol issues
  7. Review SMTP Libraries: If using SMTP libraries, ensure they are up-to-date and RFC-compliant
  8. Protocol Debugging: Enable protocol debugging when investigating 5.5.0 bounces

Technical Details

SMTP Protocol Standards

SMTP protocol is defined in:

  • RFC 5321: Simple Mail Transfer Protocol
  • RFC 5322: Internet Message Format
  • RFC 3207: SMTP Service Extension for Secure SMTP over TLS
  • Various extension RFCs for additional features

Common Protocol Issues

  • Protocol version mismatches
  • Command syntax errors
  • State machine violations
  • Extension negotiation failures
  • Authentication protocol problems

Resolution Strategies

  1. Use Standard SMTP: Follow RFC 5321 standards strictly
  2. Proper Command Sequences: Use correct SMTP command sequences
  3. Extension Negotiation: Properly negotiate SMTP extensions
  4. Error Handling: Implement proper error handling for protocol errors
  5. Testing: Test against various mail servers to ensure compatibility