Can you make a mailbox post out of cement as a deterrent for cars hitting it?

Basically, you are looking to install a crash bollard with a mailbox mounted on top.

This is a detail of one. Typically, it is restrained at the ground by pavement, but isn’t required. The bollard itself is a 4″ diameter schedule 40 metal pipe or 4″ A36 steel pipe. Some designers also add rebar (4-#4 bar vertical with #3 bar hoops at 12″ on center). (#4 bars would be 2′-0″ each and hoop would be 1′-0″ diameter for this detail.)

Keep in mind, this is not crashproof, but it is rated to prevent a large vehicle (think loaded semi or trash truck) from moving past it.