In Amberground - a land of eternal night infested by giant armored bugs - a small group of civil servants known as Letter Bees carry out their daily job. Charged with delivering important letters, each Letter Bee is partnered with a ‘Dingo' to protect them and ensure their consignment safely reaches its destination. One day, a rookie Letter Bee named Lag Seeing and his Dingo Niche are given a special assignment: they must deliver a particularly important package to a former Bee. Accompanied by an old and rather smelly ex-Dingo called Darwin who is acting as their guide, the journey to the town of Silencio is set to be difficult and dangerous. Will the rookie Lag be able to deliver the package safely, and does he even know the true purpose of his delivery?
An akuma is a soul which has been brought back to life through sadness and despair with the powers of Millenium Earl. Their mission is to terrorize the earth and all who dwell within it. The Black Order is a group of Exorcists that have sworn to cleanse the souls of the akuma, and collect the substance known as "innocence". This powerful substance can be used as an anti-akuma weapon, and ultimately can help destroy the evil Millenium Earl. Allen Walker is a young Exorcist who has the ability to tell who is human and who is an akuma with the help of his cursed eye. Together with the Black Order, Allen must find all 109 pieces of innocence to protect the world and defeat Earl.
Though D.Gray-man is about an Exorcist and Letter Bee is about a delivery Boy, both series share a similar sort of darker tone. Both also contain some comedy as well as drama and a bit of action. If you liked the tone of one, then try the other.