BOIL jars on rack in large pot filled with water 10 minutes. Place flat lids in saucepan with water. Bring to a boil. Remove from heat. Let jars and lids stand in hot water until ready to fill. Drain well before filling.
PEEL and pit mangoes. Thoroughly crush or grind fruit. Measure 4 cups into 6- or 8-quart saucepot. Add lemon juice.
STIR sugar into prepared fruit in saucepot. Bring mixture to full rolling boil (a boil that doesn't stop bubbling when stirred) on high heat, stirring constantly.
STIR in pectin quickly. Return to a full rolling boil and boil exactly 1 minute, stirring constantly. Remove from heat and skim off foam with metal spoon.
LADLE quickly into prepared jars, filling to within 1/8 inch of tops. Wipe jar rims and threads. Cover with two-piece lids. Screw bands tightly. Invert jars for 5 minutes, then turn upright. Or follow water bath method recommended by the USDA. After 1 hour, check seals.
