There is one around here, out of town, the mail box is about 15-20 ft off the ground, just below it there's a sign that says AIRMAIL
And have seen the one where the mailbox is on top of an old woodstove and it is marked as "junk mail".
Put a little WD-40 in it and let it set for a week. Might surprise you. But then, perhaps it is still best where it is now. lol