Mail addressed with the wrong zip code is delayed in delivery or routed to an incorrect location. The USPS offers a free zip code look-up website that provides a general zip plus a four-digit, route specific code, and it advises verifying a zip code before mailing a letter or parcel.
Most mail with an incorrect zip code is deliverable, but verifying the information costs the post office time and money. Aside from incorrect addresses, the postal service suggests that nothing appear beneath the address, such as a logo or slogan, because automated mail processing machines read a mailing label from the bottom up. Extraneous information that appears before the city, state and zip code slows down processing and delays delivery.