From mystery addresses and deficient postage to unclaimed items, undeliverable mail fell to the care and handling of the Dead Letter Office. For most of the nineteenth and twentieth centuries, the Dead Letter Office functioned to ensure all measures were taken to uphold the bargain that postage paid assured delivery.
Every bit of information someone wishes to communicate in a letter and the myriad of objects people purchase, gift, return, and send through the mail share the same objective—to be delivered. Failing that, the items are to be returned to sender. Failing that, they remain in the postal system. Since the 1990s, such items become the responsibility of Mail Recovery Centers. Prior to the 1990s, such lost pieces were forwarded to the Dead Letter Office.
Unclaimed mail, insufficient postage, and wrappings with nearly illegible handwriting, scant information, and simply incorrect or missing addresses were mysteries for the Dead Letter Office to solve. Clerks worked first to glean any information available on the wrapper. However, discovering the addressee or revealing information to return it to the sender sometimes necessitated opening the mail. Only the clerks of the Dead Letter Office had permission to open letters and packages. Regulations allowed them only to read the bare minimum to discover names and locations of the sender or intended recipient. In addition to ethics and reasoning skills, the work required knowledge of languages and geography. Multiple references, such as books listing common street names for U.S. cities, assisted clerks in their search.
For items never claimed or those that could not be delivered, postal workers oversaw their disposal. That included the destruction or recycling of paper from letters and wrappers. Clerks also meticulously tallied the totals for enclosed money and turned the funds over to the Treasury Department. Contents of packages were held for a period and then sold at public auction. Items that remained were destroyed or collected by the Dead Letter Office as curiosities in their museum. These objects were transferred to the Smithsonian Institution in 1911. Some objects were allocated to specific collections throughout the Institution (including the National Postal Museum) and others were deaccessioned over the century.
Cushing, Marshall Henry. Story of Our Post Office. Boston: A. M. Thayer & Co., 1893.
http://postalmuseum.si.edu/collections/object-spotlight/dead-letter-album.html (Accessed December 15, 2014.)