Errors

{
  "error": {
    "type": "domain_not_verified",
    "message": "The domain `mail.yourdomain.com` is not verified yet. Finish its DNS setup before sending."
  }
}

const data = await res.json();

if (!res.ok) {
  switch (data.error.type) {
    // The address is dead or complained. Skip it - this isn't a failure.
    case "suppressed_recipient":
      return;

    // Wait exactly as long as we're told, then try again.
    case "rate_limit_exceeded":
      await new Promise((r) =>
        setTimeout(r, Number(res.headers.get("retry-after") ?? 1) * 1000),
      );
      return retry();

    // Transient on our side.
    case "internal_error":
      return retryWithBackoff();

    // Out of allowance - alert someone rather than looping.
    case "quota_exceeded":
    case "trial_expired":
      await notifyOps(data.error.message);
      return;

    default:
      throw new Error(`${data.error.type}: ${data.error.message}`);
  }
}