Returned Letters. 
People and businesses move and die. Not everyone receives notice of the changes of address or notice of their correspondents demise. Thus, delivery of letters is often frustrated when there is no forwarding order, and should be returned to the sender. In North America, the use of a return address on an envelope is standard; in Europe, the contrary is the standard. How can this frustrated mail be returned.
The Post Office has a special section called the Returned Letter Section which is empowered to open and attempt to return the letter or package. Often, there are clues in the correspondence, and the letter is returned to sender. On correspondence containing valuables, the letter is returned by registered mail. Special envelopes of various sizes are used as appropriate (Figure 1). This service is not free, and hopefully reminds the sender that use of a return address is more economical.