The ornamental design for a mailer, as shown and described.
FIG. 1 is an unassembled perspective view of a mailer;
FIG. 2 is an unassembled top view thereof;
FIG. 3 is an unassembled bottom view thereof;
FIG. 4 is an unassembled right side elevational view thereof, of which an unassembled left side elevational view is a mirror image;
FIG. 5 is an unassembled front view thereof;
FIG. 6 is an unassembled rear view thereof;
FIG. 7 is an assembled perspective view thereof;
FIG. 8 is an assembled top view thereof;
FIG. 9 is an assembled bottom view thereof;
FIG. 10 is an assembled front view thereof; and,
FIG. 11 is an assembled right side elevational view thereof, of which an assembled left side elevational view is a mirror image.
The broken line showing of fold lines and the adhesive area are included for the purpose of illustrating environmental structure and forms no part of the claimed design.