| 401 | invalid_key | The API key is missing, malformed, or revoked. |
| 402 | billing_required | Production sending requires an active Free, Pro, or Custom workspace. |
| 403 | quota_exceeded | Monthly quota (or the pay-as-you-go safety ceiling) reached. |
| 403 | account_paused | The deliverability autopilot paused sending; check Metrics. |
| 403 | attachments_require_pro | Outbound REST attachments require Pro or Custom. |
| 403 | contact_limit_reached | Your plan's contact allowance is full. |
| 403 | domain_limit_reached | Your plan's sending or receiving domain allowance is full. |
| 403 | project_required | Choose a project with X-NoticeAPI-Project-ID or use a project-bound key. |
| 403 | project_not_allowed | The API-key grant cannot access the selected project. |
| 403 | domain_not_allowed | The API-key grant cannot use the selected sender domain. |
| 403 | insufficient_scope | The API key doesn't carry the scope this endpoint requires. |
| 404 | audience_not_found | The audience id does not exist in this workspace. |
| 404 | broadcast_not_found | The broadcast id does not exist in this workspace. |
| 404 | contact_not_found | The contact id does not exist in that audience. |
| 404 | not_found | The requested template, received email, attachment, or suppression was not found. |
| 409 | domain_not_ready | The from-domain isn't verified yet. |
| 409 | idempotency_mismatch | That Idempotency-Key was already used with a different send payload. |
| 409 | recipient_suppressed | A recipient is on your suppression list. |
| 409 | duplicate_audience | An audience with that name already exists. |
| 409 | duplicate_template | A template with that name already exists. |
| 409 | broadcast_not_draft | Broadcasts can only be sent once. |
| 409 | broadcast_not_deletable | Only draft or scheduled broadcasts can be deleted. |
| 409 | broadcast_not_schedulable | Only draft or scheduled broadcasts can be scheduled. |
| 409 | broadcast_not_scheduled | Only scheduled broadcasts can be canceled. |
| 409 | email_not_cancelable | Only scheduled transactional emails can be canceled. |
| 409 | email_not_reschedulable | Only scheduled transactional emails can be rescheduled. |
| 422 | invalid_schedule | scheduledAt is missing, invalid, in the past, or more than 30 days out. |
| 422 | invalid_payload | Validation failed; the error message says which field. |
| 422 | invalid_from | The sender address could not be parsed. |
| 422 | invalid_idempotency_key | Idempotency-Key must be at most 256 characters. |
| 422 | mixed_simulation | Simulator and real recipients can't share one send. |
| 422 | unknown_simulation | A simulator recipient must use delivered@, bounced@, complained@, or suppressed@. |
| 429 | daily_limit_reached | Warm-up ramp or plan daily cap hit; limits grow automatically. |
| 429 | rate_limited | Too many requests; respect Retry-After. |