Just to caution that you have a serious liability issue if you
do something like this and somebody gets hurt when they crash
into your purposely-armored mailbox.

I'd suggest the spring idea is a lot safer.

Don Bruder wrote:

Yep, our neighbor down the road back in Michigan had one similar to
that, but of his own design. His swung on the horizontal and reset bu
way of an angled cut and its own weight. He went one step further and
"armor-plated" it - The entire area around the mailbox itself was
enclosed in those plates that go between the rail and the tie on
railroad tracks welded up to form a seriously impressive bit of
armoring.