matrix-spec/event-schemas/examples/m.room.message.feedback

{
  "age": 242352,
  "content": {
    "type": "delivered",
    "target_event_id": "$WEIGFHFW:localhost"
  },
  "origin_server_ts": 1431961217939,
  "event_id": "$WLGTSEFSEF:localhost",
  "type": "m.room.message.feedback",
  "room_id": "!Cuyf34gef24t:localhost",
  "sender": "@example:localhost"
}