Which does the U.S. Postal Service use: ZIP code or city?

The U.S. Postal Service sorts mail by ZIP code. Within a ZIP code, mail is sorted by street address. U.S. Mail is never sorted by city.

Potential address errors are checked against street address and ZIP code to determine the most likely match. The city is disregarded in this process. Mail sent to an incorrect city but a correct ZIP code is sent to the correct destination, but mail without the correct ZIP code requires manual sorting.

In cases where neither automated sorting nor human intervention can determine the correct address, mail is returned to sender or sent to the Mail Recovery Center. If the Mail Recovery Center is unable to determine the recipient, the mail is destroyed.