I find D.Gray-man and Letter Bee quite similiar.
Lag and Allen both have special eyepower which they use on their journeys, both join an organization which makes 'em to go on journeys (Allen fightning akumas and Lag delivering letters) and the overall feeling of the series is quite similiar except that D.Gray-man is bit darker than Letter Bee.
The main protagonists are incredible similar in both looks and personalities. They both have a dark colour scheme, very well drawn uniforms for the main cast, and the overall feel and tone is similar. If you like one, you're very likely to enjoy the other.
D.Gray-man and Tegami Bachi has secret organizations, a world full of mysteries, and a main protagonist with an unique power. In fact, their unique power becomes a prominent focus in the story as we discover its hidden secrets. Both series also contain non-romantic relationships, background storytelling about the main character's pasts, and inner demons that become part of their lives.
Both animes display some similarities like, for instance, they involve Organizations and have as main protagonist a little boy who searches for his lost friend. Along the way he starts working for the organization and makes new friends while he tries to find out the truth about the disappearance of his friend.
Karneval and Tegami Bachi combines elements of action, fantasy, and mystery into a story of colorful potentials. Both series has secret organizations composed of characters of mysterious origins. There is the theme of amnesia present that has connection with the story. In addition, there is presence of supernatural forces present throughout both series. Karneval takes on a josei style approach while Tegami Bachi has the feels of an adventurous story.
About a young boy who enters a service industry (mail delivery, train conductor). Have similar sorts of worlds, with an undercurrent of class struggles. The people use magical stones (spirit amber, rainbow stone).
Kiba feels like a more adult version of Tegami. They start off with a young boy who loses his family, and is taken around by a rather stoic adult. This adult works a maligned trade (as a letter bee, arms peddler). They explain that their wares are ambiguous: they can be used for good or evil. Weapons can be used to kill, letters can be used to tell lies or hurtful truths. The boy's eyes are opened to the truth of the world he lives in. Have a similar steampunk/Western style.
Both manga series' star a 12 year-old boy who joins a large organization in hopes of finding his parent (Ging in HxH, Lag's mother in Tegami Bachi). These manga also have adventure, plenty of likeable characters, unique fantasy settings, and nice world-building that makes their worlds even more interesting.
Tegami Bachi and Shinigami x Doctor have organizations that fight against a monster who kills humanity ,and when that monster is defeated they both live behind the memories of their victim in Tegami bachi it's in form of heart and in Shinigami x Doctor is in form of pollen.
The settings are pretty similar: Letter Bee: people are corralled in 3 different areas based on class, those of higher class get to live closer towards the middle. People do not venture out of their regions due to Gaichuu which are large insects which steal peoples' "hearts". In Shingeki no Kyojin: There are 3 different areas or they call them walls. While it does not explicitly say that the areas are divided into classes, the king resides in the innermost area with the military police. The people do not venture outside the walls due to the titans that would kill them. />
There is a lot of mystery involved in Shingeki no Kyojin and the nature of the titans as well as in Tegami Bachi and the Gaichuu's as well as the capital Akatsuki. read more
Both main characters work for government so they're disliked by some people, powerful gems, loosing someone important in the childdhood.There are lot of similarities, also the plot strongly resambles Alchemist , especially Reverse and their way of acting
Both stories center on a kid who grows up to follow a profession because they admire someone they met. In Cristo, it's Towa, who admires the Orange-eyed Messiah and becomes a sweeper. In Letter Bee, it's Lag, who admires Gauche and becomes a letter bee to meet with him again.
Both main characters go on a journey to find someone while fighting with guns of power (magic). In both stories the main characters have a type of cute fighting assistant. The drawings have some similarity. The plots are both interesting; in Letter Bee he fights monsters as he journeys to find his lost friend, in Witch Hunter he fights witches as he tries to hunt down his witch-turned sister. Both characters posses unique powers that they grow into as the story moves forward.
Completely different settings and plot but Aria has some chapters about delivering letters that holds people's hearts and memories, so both manga shares the same magical image of writing and getting letters