Mail (2004)

Blurb:
Private detective Reiji Akiba has a theory about those awkward moments and weird coincidences we all encounter in life. They are actually encounters with the dead-their way of sending us a message. But you may not want to open such strange mail from beyond-not unless you can see the ghostly attachment, like Akiba can. And not unless you carry a gun that can kill what isn’t alive, like Akiba’s aptly named Kagutsuchi, "the tool between God and earth" . . . digging a divine grave to lay to rest the evil dead.

Source: www.anisearch.com/manga/2622