You can build a homemade mailbox from PVC pipe that is cheap to make and sturdy when installed.
Here is a homemade mailbox from PVC pipe design that's almost impossible to
destroy. First dig a hole about a foot deep and about a
foot across. In the center drive a steel fence post. Leave
the top of the post at the height you need the mailbox.
Over the post put a large-diameter PVC pipe and fill the
pipe around the post with concrete. Fill in the hole with
concrete so about a foot of the PVC is set. Glue a shorter
piece of curved-joint PVC to the top. Even though the front
is open, very little moisture gets in. Use a long strip of
painted wood or metal for the flag and attach it with a nut
and bolt through a drilled hole. Tighten it just enough to
keep the flag in position.
The white of the PVC makes your name and number show up
well on the box. No other maintenance is needed. You never
have to paint it, and you don't have to worry about a
wooden post rotting and having to be replaced.