The problem of communication is not rupture between spirits but letters that never arrive. With lost letters, the disposal of the dead becomes critical. They sort the letters and consign most to the flames after removing money, jewelry, or other items of value. Only in the case of obviously valuable enclosures were efforts made to return to sender, a policy in contrast to those of the United Kingdom and France. A lock of hair, of course, was a standard Victorian memento of the dead.
Dead letters stand in for the oblivion of the dead.
The symbolic association of the letter and the body is as least as old as the Torah. Dead letters are, in an Augustinian mode, emblematic of our mortal state, prone to become lost in transit. The trope of dead letters clearly plays on the Christian idea that the letter without the spirit, like the body without the spirit, is only a corpse. The Dead Letter Office deals with the materiality of communication, not its supposed spirituality. It is the dump for everything that misfires.