Exact response pattern
550 5.7.1 Service unavailable; client host blocked using Barracuda Reputation Block ListPermanentBarracudaBlocklist
What it means
The receiving gateway reports that the sending IP is on the Barracuda Reputation Block List.
Likely causes
- Poor recent reputation.
- Compromised sender.
- Bad neighboring traffic.
Recommended response
- Check the IP in the Barracuda lookup.
- Correct abuse or configuration issues.
- Submit a removal request with accurate information.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.