Aru Hi Inu no Kuni kara Tegami ga Kite
At a place in this world...there is a "Dog World" that can be visited by all dogs. The world that can be visited by dogs who still alive, for playing... and where the deceased dogs can live happily.
At a place in this world...there is a country where only dogs are authorized. All the dogs know how to go there and stay there regularly. This is here that the deceased dogs can live happily too. And those ones are allowed to send a single letter to a human of their choice.