My friends Fred and Frannie built a box in front of their mail slot, to keep the letters from going all over the floor. Into that box went Sam the Siamese. When the letter carrier came, Sam went into action, swatting through the slot, and singing as only a Siamese can. It was a couple of weeks before Fred and Frannie found out why they weren't getting any mail. Then they put a lid on the box.
The letter carrier at our house used to smack our standard poodle on the nose with a bunch of letters. Like most big poodles, ours was excessively friendly, unless a letter carrier came up on the porch. It was the only time he became aggessive.