Exact response pattern
550 5.7.1 Unfortunately, messages from [IP] were not sent. Please contact your Internet service provider since part of their network is on our block list (S775)PermanentMicrosoftReputation
What it means
Microsoft reports a block affecting the IP or nearby network range.
Likely causes
- Poor IP or network reputation.
- Abuse from the same allocation.
- Newly assigned contaminated IP space.
Recommended response
- Stop sending and audit the IP.
- Check SNDS and ownership.
- Remediate abuse.
- Request review through the appropriate Microsoft channel.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.