Puree plums in a food processor until smooth. Pour through a fine-mesh sieve set over a bowl and press on the solids to extract all the juice.
Combine port (or juice), water and sugar in a small saucepan. Heat over medium heat, stirring, until the sugar is dissolved.
Stir the syrup into the fruit puree. Chill the mixture in the refrigerator until cold, about 4 hours.
Pour the sorbet mixture into an ice cream maker. (No ice cream maker? See Tip.) Freeze according to the manufacturer's directions.
Make Ahead Tip: Freeze in an airtight container for up to 1 week. Let soften slightly before serving.
Tip: If you don't have an ice cream maker, freeze the mixture in a shallow metal cake pan or ice cube trays until solid, about 6 hours. Break into chunks and process in a food processor until smooth, stopping to scrape down the sides as needed.
160 calories;0 g fat(0 g sat); 2 g fiber; 35 g carbohydrates; 1 g protein; 7 mcg folate; 0 mg cholesterol; 32 g sugars; 17 g added sugars; 474 IU vitamin A; 13 mg vitamin C; 10 mg calcium; 0 mg iron; 2 mg sodium; 234 mg potassium