Objection 1. It would seem that it is lawful, in matters purely corporal, to communicate with an excommunicated person. For excommunication is an act of the keys. But the power of the keys extends only to spiritual matters. Therefore excommunication does not prevent one from communicating with another in matters corporal.
Objection 2. Further, "What is instituted for the sake of charity, does not militate against charity" (Cf. 11, 1, Objection 1). But we are bound by the precept of charity to succor our enemies, which is impossible without some sort of communication. Therefore it is lawful to communicate with an excommunicated person in corporal matters.
On the contrary, It is written (1 Corinthians 5:11): "With such an one not so much as to eat."
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. But, since the Church resorts to excommunication to repair and not to destroy, exception is made from this general law, in certain matters wherein communication is lawful, viz. in those which concern salvation, for one is allowed to speak of such matters with an excommunicated person; and one may even speak of other matters so as to put him at his ease and to make the words of salvation more acceptable. Moreover exception is made in favor of certain people whose business it is to be in attendance on the excommunicated person, viz. his wife, child, slave, vassal or subordinate. This, however, is to be understood of children who have not attained their majority, else they are forbidden to communicate with their father: and as to the others, the exception applies to them if they have entered his service before his excommunication, but not if they did so afterwards.
Some understand this exception to apply in the opposite way, viz. that the master can communicate with his subjects: while others hold the contrary. At any rate it is lawful for them to communicate with others in matters wherein they are under an obligation to them, for just as subjects are bound to serve their master, so is the master bound to look after his subjects. Again certain cases are excepted; as when the fact of the excommunication is unknown, or in the case of strangers or travelers in the country of those who are excommunicated, for they are allowed to buy from them, or to receive alms from them. Likewise if anyone were to see an excommunicated person in distress: for then he would be bound by the precept of charity to assist him. These are all contained in the following line: "Utility, law, lowliness, ignorance of fact, necessity," where "utility" refers to salutary words, "law" to marriage, "lowliness" to subjection. The others need no explanation.
Reply to Objection 1. Corporal matters are subordinate to spiritual matters. Wherefore the power which extends to spiritual things, can also extend to matters touching the body: even as the art which considers the end commands in matters ordained to the end.
Reply to Objection 2. In a case where one is bound by the precept of charity to hold communication, the prohibition ceases, as is clear from what has been said.
Source: Summa Theologica (New Advent)