In dutch oven or nonreactive pot, coarsely mash strawberries. Stir in lemon zest and juice and set over high heat. Stir in pectin and bring mixture to rolling boil, stirring constantly. Add sugar and boil rapidly until mixture has thickened and darkened in color, about 20 minutes. Remove from heat and skim off any foam; stir in salt. ladle jam into 2 hot, sterilized 12-oz. jars, leaving 1/4 inch headspace; seal. In water-bath canner, process jars for 10 minutes.