Sender Unknown is a special event stage. Upon first completion, it grants the player a random gift.
This stage is extremely easy; nearly any if not every Cat can beat this stage. The boss, Package Doge, is extremely weak, dealing very little damage and not moving whatsoever. Just send out fast attackers like Cow Cat and Crazed Cow to quickly kill it. The base will fall just as quickly. For an even faster clear, just use an AoE Long Distance unit like Kite or Golfer who will attack the boss but end up hitting the base, winning the level very quickly.
- This stage has the lowest base HP in the entire game.