I recently visited the National Postal Museum in Washington DC. It includes some interesting history of the US Postal Service and a statue of Ben Franklin. The architecture of the building is much like that of Union Station next door.
Included was a small exhibit about the Dead Letter Office. Many people from Europe would write to their relatives that immigrated to the USA with only city name in the address. They didn’t realize how large the cities were, and that without a proper address, the postal service could not deliver.
Nearly a century later, the post office now requires more than address to deliver mail in a reasonable time frame. First they required a zip code, then a zip+4, and now zip+4+2.
Learning: Whether it is for personal or marketing correspondence, know your recipient and completely address the envelope.