Mailer: Google / Gmail
{
  "error": {
    "code": 429,
    "message": "Resource has been exhausted (e.g. check quota).",
    "errors": [
      {
        "message": "Resource has been exhausted (e.g. check quota).",
        "domain": "global",
        "reason": "rateLimitExceeded"
      }
    ],
    "status": "RESOURCE_EXHAUSTED"
  }
}