The ornamental design for a mailbox, as shown and described.
FIG. 1 is a front perspective view of a mailbox, showing my new design, in an open configuration;
FIG. 2 is another front perspective view thereof, in another open configuration;
FIG. 3 is a side view thereof, shown in a closed configuration; and,
FIG. 4 is another front perspective view thereof, in a closed configuration.
The broken line showing of a mailbox is included for the purpose of illustrating only and forms no part of the claimed design. All surfaces not shown form no part of the claimed design.