Thank you for the image.

The sign says "Briefkasten der Stadt Östringen" ("Mailbox of the town of Östringen"), so it most likely belongs to the town hall it stands in front off.

As the Website of the Deutsche Post lists no actual post box there and as there seem to be no private postal services in the area, I'm going to remove the amenity=post_box there.

Thank you for the report, hope you're doing well.