In Canada, they have postal zipcodes that take the "letter-number-letter number-letter-number" form sort of like the UK format. Kids in Canada can write to:
Santa Claus (or Saint Nicholas, or anything like that)
1 Candycane Lane, North Pole (or North Pole, Nunavut, or really any street address)
H0 H0 H0 (This zipcode is really the only critical part, and Canadian sort-machines ignore weird spacing and kick it to H0H 0H0)
The letter is machine-sorted and sent to some government agency somewhere, where a team of "Santas" spend the season writing back individualized letters to the kids. It's clever because of course the North Pole is closer to Canada than to anywhere else and as everyone knows, Santa lives in the North Pole. Obviously if there were anywhere you could mail him from iw would be Canada.
Although... in the US, I know it's an individualized system from P.O. to P.O.. The Postmaster of each office decides what to do with the unmailable letters that arrive arround Christmastime. Letters that are addressed simply to "Santa" or something are collected. I worked at a P.O. for a few years, and at the place I worked, the Postmaster used to take his job very seriously. He was a scrawny old man with a big beard and I'd always see him in there bent over his writing desk in the cage. I think he hand-wrote the responses. Maybe he was Santa.