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Troubleshooting Gmail 550 5.7.25 Error: PTR Record Mismatch for Sending IP

25 reverse DNS failures, focused on PTR alignment, SMTP hostname evidence, DNS ownership, and safe provider-side changes.

Quick Read

  • Symptom: 25 reverse DNS failures, focused on PTR alignment, SMTP hostname evidence, DNS ownership, and safe provider-side changes.
  • Check first: Verify the PTR record for the sending IP.
  • Risk: Changes system state

Symptoms

Gmail returns a 550 5.7.25 error indicating a reverse DNS mismatch despite a PTR record existing for the sending IP.

Environment

Linux-based mail servers, DNS management systems, Gmail email service.

Most Likely Causes

The PTR record for the sending IP does not match the hostname used in the SMTP transaction, or the DNS records have not propagated correctly.

What to Check First

  1. Verify the PTR record for the sending IP.
  2. Check the hostname used in the SMTP transaction.
  3. Ensure the sending IP is not blacklisted.

Fix Steps

  1. Check the PTR record for the sending IP.

    Use the 'dig' command to verify the PTR record.

    Example pattern only. Adjust for your environment before running.

    dig -x <sending_IP>
  2. Check the hostname used in the SMTP transaction.

    Review the mail server configuration to confirm the hostname.

    Example pattern only. Adjust for your environment before running.

    cat /etc/hostname
  3. Update the PTR record if it does not match the hostname.

    Modify the DNS settings to ensure the PTR record matches the hostname.

    Example pattern only. Adjust for your environment before running.

    <DNS management command to update PTR record>
  4. Wait for DNS propagation.

    Allow time for DNS changes to propagate, which can take up to 48 hours.

  5. Test sending an email to Gmail again.

    Attempt to send an email to a Gmail account to verify if the issue is resolved.

    Example pattern only. Adjust for your environment before running.

    echo 'Test email' | mail -s 'Test' <recipient@gmail.com>

Validation

  • Confirm that emails are being delivered to Gmail without errors.
  • Check the mail logs for any further issues.

Logs to Check

  • /var/log/mail.log
  • /var/log/maillog

Rollback and Escalation

  • If the PTR record update causes further issues, revert to the previous PTR record.

Escalate When

  • If the issue persists after DNS propagation, escalate to the network team to investigate DNS settings.

Edge Cases

  • Multiple PTR records for the same IP can cause issues.
  • Firewall settings may block SMTP traffic.

Notes from the Field

  • Ensure the mail server's hostname is properly set and matches the PTR record.
  • Monitor DNS changes as they can take time to propagate.