Thomas Aquinas

“ I answer that, Excommunication is twofold: there is minor excommunication, which deprives a man merely of a share in the sacraments, but not of the communion of the faithful. Wherefore it is lawful to communicate with a person lying under an excommunication of this kind, but not to give him the sacraments. The other is major excommunication which deprives a man of the sacraments of the Church and of the communion of the faithful. Wherefore it is not lawful to communicate with one who lies under such an excommunication. ”

“ Further, no one should incur a major excommunication save for a mortal sin. Now according to the law (Can. Praecipue, seqq., caus. xi) a man may incur a major excommunication for holding communion with an excommunicated person. Therefore it is a mortal sin to hold communion with one who is excommunicated.
On the contrary, None can absolve a man from mortal sin unless he have jurisdiction over him. But any priest can absolve a man for holding communion with those who are excommunicated. Therefore it is not a mortal sin. ”

“ But since it seems very hard that a man should be guilty of a mortal sin by uttering just a slight word to an excommunicated person, and that by excommunicating a person one would endanger the salvation of many, and lay a snare which might turn to one's own hurt, it seems to others more probable that he is not always guilty of a mortal sin, but only when he holds communion with him in a criminal deed, or in an act of Divine worship, or through contempt of the Church. ”