Just about all Zip codes in Massachusetts begin with 02 or 01. All zip codes in Vermont begin 05. The Internal Revenue Service (IRS) service center in Andover, MA, has it’s own zip code. That’s understandable, except that their zip code is 05501. Why do they get a “Vermont” Zip Code if they’re in Massachusetts?
Well, I took a quick peek at the US Post Office ZIP Code Lookup Engine and apparently the 05501/05544 ZIP code associated with the Andover IRS Center is “unique”, which I take to mean it was assigned to the Center because the Post Office wanted it that way.