10027
10027 is basically Columbia's zip code. Notably, Lerner Hall falls into this zip code, so all students with a Lerner mailbox have an address of the form: #### Lerner Hall, New York, NY 10027.
In order to ensure proper delivery, however, it is best for students to be more explicit about where their mail is going. Either including "2920 Broadway" (Lerner's address), "c/o Columbia University," "116th and Broadway," or Lerner Hall's own zip code extension, 7164, seem to work.
South of the campus (including Frat Row), all buildings fall into the 10025 zip code. So if you're using Google to look for something nearby, make sure to search "near 10025" instead of "near 10027." Unless you want to end up at a Duane Reade in the middle of Harlem.
