Mailboxes come in a variety of styles and sizes. Most are made from thin sheet metal, resin plastic or steel. But the mailbox door endures a lot of use and abuse. In time, the mailbox door can become loose and fail to stay shut. Worse yet, there might be gaps between the door and box that allows rain or snow inside the box. Plastic mailboxes will require replacement of the door when this happens. However, you can tighten a mailbox door if it is made of sheet metal or steel.
Open the mailbox door. Grasp the metal hasp on the top edge of the mailbox with a pair of needle-nose pliers.
Bend the hasp down an 1/8th of an inch with the pliers. Close the mailbox door. Squeeze the door hasp and mailbox hasp together with the pliers. This may help resolve the issue.
Or, open the mailbox door halfway and attempt to bend it straight with your hands. Place your hands on each side of the door and bend it as necessary to straighten the door. Bend the door edges toward the inside of the door with the pliers and shut the door.
Loosen the hinge bolts if you have a loose door. The mounting bolts are at the base of the mailbox. Loosen them with a wrench and screwdriver. Adjust the tabs as necessary and retighten the bolts.