Consultation and backbiting others
The act of talking about one person to another, if it is a matter of consultation for something that concerns him, then it is appropriate to do so. But if the speech is a slander and shows the flaws of others, then it is backbiting. Both the speaker and listener are sinners. A Muslim should advise the person concerned directly.
As for disclosing what is in the mind, this is determined by what is disclosed. If it is an insult to others and a statement of their flaws, there is no doubt that this is sinful, and a Muslim must suppress his anger if he has been harmed by a person, and turn to Allah Almighty for forgiveness. However, if it is a matter of reproach, whether he is in the presence of those who he alleged, or he complain to someone who can advise the other person, then this is allowed.