BRITISH posties do not grok the concept of external letterboxen, according to Miss von Brandenburg who has one.
My Lincolnshire correspondent's postie seems to manage just fine. The shite couriers make up for it by regularly failing to find the house.
Meanwhile, our door got upgraded to landlord-issue UPVC composite a couple of years ago, and with it came a generously sized letterbox
[1] with some really vicious brushes, undoubtedly so as not to compromise the door's eco rating. The postie seems to cope (about 2/3 of the doors on the street are of similar design), and it discourages the spammers from rolling their leaflets into a tube and leaving them wedged in the flap, as would happen with the previous small-but-mostly-harmless arrangement.
Postman Piers (who hasn't been a postman for many years now) was not a fan of poorly designed letterboxes, the worst type being those with a dog on the other side.
[1] Why do we call it a letterbox when it's just a letterflap? And why is a letterbox for incoming post, and a postbox for outgoing?