To a first approximation, ZIP codes are post offices. My understanding is that sorting starts with the postal codes, but I'm not clear exactly how much sorting happens where. For instance, the first three digits of the post code denotes a regional central processing facility. When 123xx has mail for 456xx, do they sort it before or after sending it to 456xx? Does the central facility for 123xx sort the mail for 12345, or just send it all to the post office? I could see it going either way.