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SMTP Bounce Code 5.4.2: Bad Connection (Permanent)

SMTP bounce code 5.4.2 indicates that a connection to the destination mail server cannot be established due to a permanent connection failure. This is a permanent failure (hard bounce), meaning there's a persistent issue preventing connection establishment.

What Does 5.4.2 Mean?

The enhanced status code 5.4.2 follows the SMTP Enhanced Status Code format:

  • 5 = Permanent failure (hard bounce)
  • 4 = Network and routing status
  • 2 = Bad connection

When you receive a 5.4.2 bounce, it means the mail server cannot establish or maintain a connection with the destination server due to a permanent issue. This differs from 4.4.2 (temporary) in that the connection problem is persistent 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 the connection issue

Common Causes

  1. Permanent Network Issues: Permanent network problems preventing connection establishment
  2. Port Permanently Blocked: SMTP ports (25, 587, 465) are permanently blocked by firewall
  3. TLS/SSL Configuration Mismatch: Permanent TLS/SSL configuration issues preventing secure connections
  4. Protocol Incompatibility: Permanent protocol incompatibility between servers
  5. Firewall Rules: Permanent firewall rules blocking or terminating connections
  6. Network Infrastructure Issues: Permanent network infrastructure problems
  7. IP Address Blocking: Your sending IP address is permanently blocked by the recipient's server
  8. Connection Policy: Permanent connection policies that prevent connection establishment
  9. Server Configuration: Permanent server configuration issues preventing connections
  10. Network Path Issues: Permanent routing or network path problems

How to Resolve

For Email Marketers

  1. Remove from List: Immediately remove the email address from your active mailing list
  2. Contact Recipient: If possible, contact the recipient through alternative channels to verify their mail server configuration
  3. Document the Issue: Document which domains are affected by permanent connection failures
  4. Check Sender Reputation: Verify your sending IP isn't blocked by the recipient's server
  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 5.4.2 bounces from your database
  3. Check Connection Configuration: Review connection settings, ports, and TLS/SSL configuration
  4. Verify IP Status: Check if your sending IP is blocked by the recipient's server
  5. Suppression List: Add these addresses to a suppression list to prevent future send attempts
  6. Log Connection Issues: Track which servers have permanent connection problems
  7. Test Alternative Ports: If possible, test if alternative SMTP ports work

Investigation Steps

When you receive 5.4.2 bounces, investigate:

  1. Test Connection: Try to manually connect to the mail server to verify the issue
  2. Check Firewall Rules: Verify if firewall rules are blocking connections
  3. Verify TLS/SSL: Check TLS/SSL configuration compatibility
  4. Test Alternative Ports: Try connecting on different SMTP ports (25, 587, 465)
  5. Check IP Blocking: Verify if your sending IP is blocked
  6. Contact Support: Contact the recipient's IT department if possible

Related Bounce Codes

Examples

Example Bounce Message

550 5.4.2 Bad connection
Connection cannot be established due to permanent configuration issues.

Example Enhanced Status Code

550 5.4.2 <[email protected]>: Bad connection - port permanently blocked

Common Email Provider Responses

  • Generic: "550 5.4.2 Bad connection"
  • Port Blocked: "Connection blocked - port not accessible"
  • TLS Error: "Permanent TLS configuration mismatch"
  • Firewall: "Connection blocked by firewall policy"

Best Practices

  1. Never Retry: Don't attempt to resend emails to addresses that bounce with 5.4.2—they will continue to fail
  2. Remove Immediately: Remove addresses with 5.4.2 bounces from your mailing list immediately
  3. Check Configuration: Review your connection configuration for compatibility issues
  4. Use Suppression Lists: Maintain suppression lists to prevent sending to servers with connection issues
  5. Monitor IP Status: Monitor if your sending IP addresses are being blocked
  6. Test Connections: Periodically test connections to verify server accessibility
  7. Document Issues: Keep records of domains with permanent connection problems
  8. Contact Administrators: If needed, contact server administrators to resolve configuration issues