A mailbox post is a pretty simple structure. You need a vertical post to go in the ground and a horizontal piece to support the mailbox.
The way you join them can make the structure both sturdy and attractive.
A half-lap joint is a great way to do that with dimensional lumber:
• Using a circular saw, cut lines half the thickness of the 4×4 post and exactly the width of the post at the point where the vertical post where the horizontal piece will cross it.
• Make cuts every eighth inch or so in between those lines.
• Chip out the remaining wood by tapping with a hammer.
• Repeat this process on the horizontal piece.
• Fit the two notches into each other at a perfect 90-degree angle. Use a few deck screws to keep them in place.
Although not necessary, you can add diagonal support under the side that will support the mailbox.
Finally, cut a piece of three-quarter-inch-thick dimensional lumber to fit into the recess under the mailbox. This will make it easier to attach the box itself to your sturdy new mailbox post.
Watch the video for detailed instructions.