Most large buildings and complexes get their own zip codes, so the World Trade Center was not unique in having its own zip code (I'm surprised it only had one actually). The zip code still gets a sizable amount of mail each day. In the weeks after the Saudi attack I read about the postal workers who used to deliver mail at the WTC, who were now virtually delivering the mail in an area of the GPO where they had recreated the various mail zones of the WTC. They'd sort the mail and "deliver" it to containers for the specific companies, who then may or may not have sent someone to pick up the mail. Anyway, according to NYC: Twilight Zone for ZIP Code at Ground Zero, there's still 3-4000 pieces of mail arriving today destined for seven buildings which don't exist.