Drill 2 holes in the side of the mailbox slightly slanted ( / ).
Screw through the inside of the mailbox into the column.
Use Fender washers on the screws.
You haven't indicated what material the column is made of. That will effect the choice of screws and possible anchors, but the process will be the same.
( Provide more info in the material and I will edit the answer to reflect the proper choices.)