Believe it or not, its a Dutch mailbox.
The postal service in Knokke-Heist has a crusade against 'problem' mailboxes. Most bad boxes are 'unmarked, too small, or difficult to reach'. But some are just... 'non-conforming'.
The best part? The postal union talks to the owner in person to tell them that they won't be able to deliver mail to that bad box anymore. But if they can't contact them within 10 days?... Then they will 'be informed by letter'.
link via: Oro Nieuws (Dutch)