The ornamental design for mailbox, as shown and described.
FIG. 1 is a front plan view of the mailbox according to the present invention;
FIG. 2 is a rear plan view of the mailbox shown in FIG. 1 according to the present invention;
FIG. 3 is a bottom, right, side view of the mailbox shown in FIG. 1 according to the present invention, the left side is a mirror image thereof; and,
FIG. 4 is a front, right side perspective view of the mailbox according to the present invention, the front, left perspective side is a mirror image thereof.