Several of them can be, actually. There is one kind of mushi called uro which burrows holes in space--certain silk-spinners capture them within coccoons, and they will travel between that coccoon and another wound from the same thread, which travelling mushishi carry with them. Uro take anything in the same 'room' with them, so if a letter is put inside the coccoon it will eventually be delivered to the mushishi in question--which is useful, since one is never sure where to send a letter the normal way so that it would reach them.