Errors

Every error is JSON with the same three fields, and the HTTP status matches statusCode:

{
  "name": "validation_error",
  "message": "from and to are required",
  "statusCode": 400
}

name is the stable machine-readable identifier; branch on it, not on message.

Common errors

Status	name	Meaning
400	validation_error	A request field is missing or malformed; message says which
401	missing_api_key	No Authorization: Bearer ss_… header
403	invalid_api_key	The key is wrong or revoked
403	domain_not_verified	Sending from an unverified domain outside test mode
403	domain_not_owned	The from-domain is verified by another team; claim it
403	sending_paused_reputation	Bounce/complaint rates tripped the safety hold; contact support
404	not_found	No such resource on your team
409	idempotency_in_progress	A request with this Idempotency-Key is still in flight; retry shortly
429	rate_limit_exceeded	Over the rate limit; back off and retry
429	quota_exceeded	Monthly sending quota reached; sending is paused until next month or an upgrade
500	internal_error	Something broke on our side; the X-Request-Id response header identifies the request for support

Internal detail (SQL, provider errors) never appears in message; it’s logged server-side against the request id.