Do not treat all 550 responses equally
Use timestamps, queue IDs, remote responses, source IP, VMTA, sender domain and message class to establish the failure boundary. Avoid reacting to one isolated delivery attempt.
Classification groups
- Unknown recipient
- Policy or content rejection
- Authentication or alignment failure
- Blocked source IP/domain
- Relay or routing error
Action
Use the enhanced code and text. Suppress invalid recipients immediately; route policy and reputation failures into incident handling instead of recipient suppression.
Operational rule: collect evidence before changing policy, make one controlled change, verify the result, and retain a rollback path.