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 theand wood screws.
You haven't indicated what material the column is made of I like Teks screws (Even though they are rated as metal screws. That will effect the choice) I find they are great for this type of screwsapplication and possible anchors, but the process will be the samevery easy to screw.
( Provide more info in If needed use a fender washer or 2 between the materialmailbox and I will editcolumn on the answerupper screw, to reflectadjust for the proper choicestaper.)