Pound the chicken breasts with a mallet between 2 sheets of wax paper until they are about 1/2 inch think or slice them in half horizontally.
In a zip lock bag mix together soy sauce, orange juice, ginger, garlic, sesame oil, and black pepper. Add the chicken and mix to coat. Seal the bag and marinate for at least 4 hours in the refrigerator.
When you are ready to grill the chicken. Remove the chicken from the marinade and let any excess marinade drip off. Preheat the grill to medium-high heat. Place the chicken onto the grill and cook for 3-5 minutes per side or until the chicken is fully cooked, the internal temperature has reached 165 degrees F.
Prepare the salsa by combining chopped mango, red onion, cilantro and lime juice in a small bowl. Season with salt and pepper and mix well. Serve over the grilled chicken.