To repair a damaged mailbox, bring it to the proper shape, if the box is out of shape. If the door does not shut properly or the post is damaged, replace the parts. Attach a hinge to one of the bottom sides of the mailbox to prevent denting.
If the box is dented or bent, tap it gently with a rubber mallet or hammer wrapped with a piece of cloth to bring the metal into its original position. If the mailbox's door does not shut smoothly, remove any dents using a mallet or hammer. Alternatively, replace the hinges, the door or the entire mailbox.
If the box's post is broken or wobbly, remove it from the ground using a car jack. Purchase a new post between 41 and 45 inches in length, and treat it for termites and rotting. Make the existing hole in the ground bigger using a shovel, layer gravel at the bottom of the hole, fix the post in it, and close the hole by packing in dirt. Attach the mailbox to the replacement post using screws.
If the mailbox is in a place where it is vulnerable to denting, affix a wooden board of the same dimensions as the box onto the mailbox post. Screw in a hinge as long as the box's length to that side of the board and the box that lies opposite to the side that gets bumped into. This causes the box to tip over when hit.