A dead letter is used when a man, most likely a soldier, is not sure of his survival with important documents or information. He will then, transfer the documents to anyone he trusts before he dies, IF he dies. i.e. "dead" letter.
Also it is a letter that cannot be delivered to its recipient, nor returned to the originator.
A letter viewed in its nominal and more suggestive light, conveys the impression of good news, cheering suggestions, pleasant surprises, or perhaps the reverse of these, bad tidings, woe, sorrow; but always some stirring and awakening impulses are to be derived out of a letter--whether they be good or ill, for pleasure or for pain.
Many letters, thus returned, find their owners here, and the little pittance, which thoughtful friends had intended to aid in bringing then out of the land of starvation to that of promise and of plenty, is drawn at the counters of the bankers who issued the draft.
The letters are opened, read, stamped with the stamp of the D. Bureau, and entered, by number, under the letter in the alphabet, corresponding with the initial of,the surname, of the addressed.
Once opened, the contents of letters were considered sacred, so much so that the deadletter clerks wereand still areforbidden to read any more of the communications than absolutely necessary to determine where the letters should go.
At the end of the 19th century it was not uncommon for the clerks in the DeadLetter Office to handle as many as 23,000 pieces of "dead" mail daily.
On an average, over 80,000 of the letters and packages forwarded to the DeadLetter Office at that time didn't contain any address at all, and on much of the rest, the information provided was either incomplete or so poorly written that the addressee could not be found.