Tegami Bachi

This is a really good anime, I liked it but i think it was a bit to sorrowful and kinda depressing,  its a bout a kid named Lag Seeing who's mother suddenly went missing when he was a child, and she is apparently in the capital of their dark world.  Lag is delivered as a package with the help of a Letter Bee, Gouche Suede who later looses his heart when Lag finally makes his dream of becoming a Bee but he later tries to solve the mystery of how Gouche lost his heart.