Classification
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.
450 commonly indicates a temporary mailbox or policy condition. Keep it in the retry path unless repeated evidence and provider documentation justify a different treatment.
Common causes
- Greylisting or temporary policy enforcement
- Mailbox locking or transient storage conditions
- Recipient verification service unavailable
- Rate or reputation pressure represented as 450
Operational handling
Track recurrence by recipient domain and enhanced code. Suppress only after your documented retry horizon or a later permanent response.
Operational rule: collect evidence before changing policy, make one controlled change, verify the result, and retain a rollback path.