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SMTP Bounce Code 5.4.3: Routing Server Failure (Permanent)

SMTP bounce code 5.4.3 indicates that an intermediate routing server has permanently failed and cannot forward messages. This is a permanent failure (hard bounce), meaning there's a persistent issue with the routing infrastructure that prevents message delivery.

What Does 5.4.3 Mean?

The enhanced status code 5.4.3 follows the SMTP Enhanced Status Code format:

  • 5 = Permanent failure (hard bounce)
  • 4 = Network and routing status
  • 3 = Routing server failure

When you receive a 5.4.3 bounce, it means an intermediate mail server in the routing path has permanently failed and cannot forward your message. This is different from 4.4.3 (temporary) in that the routing server failure is permanent and won't resolve.

Bounce Type

  • Type: Hard bounce (permanent failure)
  • Category: Network/Routing
  • Action Required: Remove the email address from your mailing list or investigate routing alternatives

Common Causes

  1. Routing Server Permanently Down: An intermediate routing server has been permanently shut down
  2. Server Decommissioned: The routing server has been decommissioned and is no longer operational
  3. Permanent Configuration Issues: Permanent misconfiguration of routing rules or server settings
  4. DNS Permanently Invalid: DNS records point to routing servers that no longer exist
  5. Network Infrastructure Failure: Permanent network infrastructure problems affecting routing
  6. MX Record Issues: MX records point to permanently unavailable routing servers
  7. Routing Server Migration: Routing server migration complete with old servers permanently offline
  8. Abandoned Routing Infrastructure: The routing infrastructure has been abandoned
  9. Permanent Resource Exhaustion: Routing server has permanently exhausted resources
  10. Critical Routing Failure: Critical failure in routing infrastructure that cannot be resolved

How to Resolve

For Email Marketers

  1. Remove from List: Immediately remove the email address from your active mailing list
  2. Verify Domain Status: Check if the domain and routing infrastructure are still active
  3. Contact Recipient: If possible, contact the recipient through alternative channels to verify their mail routing
  4. Document the Issue: Document which domains are affected by permanent routing failures
  5. Monitor for Recovery: Periodically check if the issue has been resolved (though unlikely)
  6. 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 5.4.3 bounces from your database
  3. Check MX Records: Verify MX records to identify routing server issues
  4. Suppression List: Add these addresses to a suppression list to prevent future send attempts
  5. Monitor Domain Status: Periodically check if domains with 5.4.3 errors have resolved their issues
  6. Log Routing Patterns: Track which routing servers are permanently failing
  7. DNS Validation: Validate DNS and MX records for affected domains

Investigation Steps

When you receive 5.4.3 bounces, investigate:

  1. Check MX Records: Verify if MX records are valid and pointing to active routing servers
  2. Test Routing Path: Try to trace the routing path to identify failed servers
  3. Verify Domain Status: Check if the domain is still active and routing is configured
  4. Contact Support: Contact the recipient's IT department if possible
  5. Document Pattern: Document if multiple addresses from the same domain are affected
  6. Check Alternative Routes: See if alternative MX records provide working routes

Related Bounce Codes

Examples

Example Bounce Message

550 5.4.3 Routing server failure
Intermediate routing server permanently unavailable.

Example Enhanced Status Code

550 5.4.3 <[email protected]>: Routing server failure - intermediate server permanently down

Common Email Provider Responses

  • Generic: "550 5.4.3 Routing server failure"
  • Server Down: "Intermediate routing server permanently unavailable"
  • MX Error: "Mail routing path includes permanently failed server"
  • Infrastructure: "Routing infrastructure permanently unavailable"

Best Practices

  1. Never Retry: Don't attempt to resend emails to addresses that bounce with 5.4.3—they will continue to fail
  2. Remove Immediately: Remove addresses with 5.4.3 bounces from your mailing list immediately
  3. Verify MX Records: Check MX records to understand routing configuration
  4. Use Suppression Lists: Maintain suppression lists to prevent sending to domains with routing failures
  5. Monitor Domain Health: Track domains with permanent routing infrastructure issues
  6. Check Alternative Routes: Verify if alternative MX records provide working routes
  7. Document Issues: Keep records of domains with permanent routing failures
  8. Contact Administrators: Contact domain administrators if routing issues affect important communications

Technical Details

MX Record Analysis

When investigating 5.4.3 bounces:

  • Check MX record priorities
  • Verify all MX records are valid
  • Test connectivity to each MX record
  • Identify which routing servers have failed

Routing Path Analysis

Understanding the routing path:

  • Trace the path from your server to destination
  • Identify intermediate routing servers
  • Determine which server in the path has failed
  • Check if alternative routes exist