Exact response pattern
550 5.7.1 Service unavailable, Client host blockedPermanentMicrosoftReputation
What it means
Microsoft is refusing the sending IP under a reputation or policy block.
Likely causes
- Compromised credentials.
- Poor IP reputation.
- Spam complaints.
- Unwanted traffic.
Recommended response
- Stop questionable traffic.
- Secure credentials and audit senders.
- Use SNDS and the Microsoft delist process after remediation.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.