/summa/XP/XPII/XP_Q23:
We must now consider communication with those who are excommunicated. Under this head there are three points of inquiry:
(1) Whether it is lawful to communicate in matters purely corporal with one who is excommunicated?
(2) Whether one who communicates with an excommunicated person is excommunicated?
(3) Whether it is always a mortal sin to communicate with an excommunicated person in matters not permitted by law?