curl -X GET \
-H "QB-Token: bdbb2bd5abb81b1d7fe7be03db9305b46e012bba" \
https://api.quickblox.com/users/by_email.json?email=alex@gmail.com

{
  "user": {
    "id": 96754523,
    "full_name": "Alex Miller",
    "email": "alex@gmail.com",
    "login": "Alex",
    "phone": "+3123555",
    "website": "https://mysite.com",
    "created_at": "2019-08-27T21:08:55Z",
    "updated_at": "2019-08-27T21:08:55Z",
    "last_request_at": null,
    "external_user_id": 126672,
    "facebook_id": "888766",
    "twitter_id": "982366",
    "blob_id": null,
    "custom_data": null,
    "age_over16": true,
    "allow_statistics_analysis": true,
    "allow_sales_activities": true,
    "parents_contacts": "",
    "user_tags": "android,web"
  }
}

{
  "errors": {
    "base": [
      "Required session does not exist"
    ]
  }
}

{
  "errors": {
    "base": [
      "Users retrieving forbidden in admin panel, application settings. Setup it here https://admin.quickblox.com/apps/{app_id}/service/users/settings"
    ]
  }
}

{
  "code": null,
  "message": "Couldn't find user"
}