Look up a zip code here.

The extra four letters on the end is a more recent addition, so not all addresses include it. I have a post office box for my mailing address which causes a lot of confusion. Like Kendall said, if a postal employee has had a bad day (or if they're new) and the letter has the street address instead of the PO box number, it gets returned or has a nasty sticker on the outside saying that I should advise all correspondents of my correct address. This is impossible for me to do because people get my street address out of the phone book. Also, every time I fill out a form, I put both addresses, but some bureaucrat deletes the PO box number so that only my street address is entered. Here in a small town you can actually just put the name and town, and it will eventually get to the correct person.