Repairs and upkeep to the common areas are usually covered by the condo association. That's part of what your monthly fees cover. If the repairs are due to a storm, or damage caused by a vehicle etc, perhaps the condo insurance will provide coverage for the repair.
Unless it is otherwise specified in the condo's bylaws, they association should be responsible for the maintenance and upkeep of the common areas. Contact your HOA representative to report teh broken mailbox. If he maintenance company is offsite they may not be aware it is broken.