Rinse clean jars with twist-off lids® with hot water and allow to drain on a clean kitchen towel. Wash the plums, drain, core, finely chop and weigh 1 kg. Mix the plums and lemon juice in a saucepan with the preserving sugar. Bring everything to the boil while stirring over high heat. Puree and then simmer for at least 3 minutes, stirring constantly. Stir in cinnamon. Remove from the heat. If necessary, skim and fill the prepared glasses immediately to the brim. Close with the twist-off lids®, turn over and let stand on the lid for approx. 5 minutes. Let cool down completely.