Get mailcow statistics

Example response

{
  "code": 0,
  "data": {
    "domain_count": 5,
    "mailbox": {
      "monthly": {
        "unused_count": 75,
        "used_count": 25
      },
      "total_count": 100,
      "yearly": {
        "unused_count": 75,
        "used_count": 25
      }
    },
    "total_count": 100,
    "used_count": 50
  },
  "desc": ""
}