Fifty years ago, I was given a coin bank styled after the red Danish Post letter box. That was in Solvang, CA. As these Danish immigrant-character communities (look also in Elkhorn, Racine, Greenville, etc) are little time capsules, you may have to travel to America to find a replica red slot to drop your letter.

The article wasn't clear how letters from outside Denmark will be handled, but maybe that's implicit in the Dao contract.

EDIT: maybe Royal Mail was never the Danish term, but I thought it was on a Lego set too...