The post office will send it to the central warehouse, I suspect that he was in Pilsen for some time. There they open the undeliverable consignment and try to find out the addressee from the contents. If they fail to do so, they will keep it in the archives for some time, in case the addressee starts looking for his shipment. When this doesn't happen, shredding starts. They cut the letter into strips and pour it into the nurses for recycling.