319. Changing Utensils Between Meat and Dairy

46:15 The practice is not to kasher dairy utensils for use with meat, or vice versa. (See Mishnah Brurah 509:25 for a discussion of opinions in this matter. If one kashers a utensil for Pesach, he may incidentally switch it from meat to dairy or vice versa – MB 451:19.)

46:16 If one sends wine, meat, or pieces of fish that do not have visible signs of being kosher by a messenger who is not Jewish, or by a non-observant Jew, the package must be closed with two seals. When sending mevushal wine (wine cooked above a certain temperature), wine vinegar, milk, bread or cheese, one seal is sufficient.