If necessary, remove the cement base from the old post. Dig a new hole using a post-hole digger. According to standard postal service codes, a mailbox should not sit any higher than 45 inches from street level.
Place the post in the hole. Level it and attach two support beams at the center to keep the post standing up straight. Place one at the side of the post, and then add another to the back. This will ensure the post does not move while pouring the concrete and it remains level while the concrete is drying.
Pour the dry concrete straight into the hole leaving 3 to 4 inches of space from the top of the hole. In most cases a 50-pound bag of quick-dry concrete should suffice. Saturate the concrete. For a 50-pound bag you will need about a gallon of water. Be sure to leave support beams in place for 4 to 6 hours or until concrete is dry.
Most mailbox kits will come with a wood anchor to attach to the post. After screwing the anchor to the post, slide the mailbox over anchor and secure with screws.