The ornamental design for a mailbox flag, as shown and described.
FIG. 1 is a right side elevational view of a mailbox flag showing my new design in a down or "no mail" position, the broken lines are for illustrative purposes only and form no part of the claimed design, the undisclosed left side is not part of the claimed design;
FIG. 2 is a right side elevational view similar to FIG. 1 but with the flag in the up "mail delivered" position; and,
FIG. 3 is a front elevational view of the flag, the rear view being a mirror image thereof.