SMTP code 451 4.7.550
What the knowledge base's own tables say about this code, and which articles say it — a description of the reply, never a placement prediction.
- Ticket
- "New IP: mail suddenly rejected / deferred"
- Playbook
- A or B
- Test, finding and fix
- Read the text. 5.7.1 citing the IPv6 guidelines means the authentication and PTR requirements in IPv6 sending. 451 4.7.550 from Microsoft means a throttle on a new IP address: let it ramp up, which takes weeks. Greylisting means retrying from the same IP address
From Delivery Troubleshooting Playbooks · Scenario library (worked examples)