| 400 | invalid_mailbox_name | Mailbox name doesn’t meet format requirements |
| 400 | missing_idempotency_key | Send endpoint requires Idempotency-Key header |
| 401 | unauthorized | Invalid or missing API key |
| 403 | feature_requires_paid_plan | Paid-only feature used on free plan (e.g. external replyTo) |
| 403 | forbidden | A scoped key reached outside its pod or inbox, or attempted an action its scope doesn’t allow |
| 404 | not_found | Resource doesn’t exist or doesn’t belong to your account |
| 409 | address_taken | Mailbox name or address already in use |
| 422 | recipient_suppressed | Recipient is on the suppression list |
| 422 | api_key_limit_reached | Pod or inbox already has 20 active API keys |
| 429 | rate_limit_exceeded | Too many requests - check Retry-After header |
| 429 | cold_outreach_limit | Cold send limit exceeded for new inbox |
