The ornamental design for a mailbox, as shown and described.
FIG. 1 is a perspective top and front view of a mail box showing our new design, the undisclosed rear is not part of the claimed design;
FIG. 2 is a front view thereof; and,
FIG. 3 is a side view of one side thereof, the other side being a mirror image.
The broken line(s) that define the shaded area of the mailbox disclose the bounds of the claimed design and form no part thereof; the broken lines to the base of the mail box are included for the purpose of illustration and form no part of the claimed design.