The circumstances in which backbiting is permissible

Question:
Sister Sarah from Marath sent this question. She says: Honorable Shaykh, one of the sisters asks: If I speak about someone in front of a group who do not know him, either praising him or commending him, or the opposite, does that count as backbiting if I do not mention his name?

Answer:
As for commending him and praising him for what is in him, that is a good thing. As for blaming him and mentioning the faults in him, that is not permissible because it is backbiting. Backbiting is not permissible except in the exceptional circumstances, such as complaining to a judge, saying, "So-and-so has wronged me, he ate my right, he oppressed me," and the like, in order to seek justice against him; in that case, it is permissible. Likewise, forbidding evil: you go to the one in authority, to the deputy of the one in authority, to the men of the hisbah (public order), to the head of the hisbah, and you say, "So-and-so does such and such," or "So-and-so does not attend the congregational prayer," in order that they may take action against him. In these three circumstances, backbiting is permissible because it is for a valid purpose. Anything other than that is not permissible. As for their saying that there is no backbiting regarding an unknown person, that has no basis. Backbiting is not permissible except in these matters. Yes.