The ornamental design for a mailboxalertflag, as shown and described.
FIG. 1 is a perspective view of a mailbox alert flag, as shown on an existing mailbox and in a deployed state;
FIG. 2 is an enlarged top plan view thereof;
FIG. 3 is an enlarged bottom plan view thereof;
FIG. 4 is an enlarged front view thereof;
FIG. 5 is an enlarged rear view thereof;
FIG. 6 is an enlarged side view thereof; and,
FIG. 7 is a perspective view of a mailbox flag, as shown mounted on an existing mailbox and in a resting state.
The broken line showing of a mailbox and post is included for the purpose of illustrating environmental structure and forms no part of the claimed design.