Hadhrat Thanvi (may Allah be pleased with him) said;

“If there is a genuine religious need then Gheebat is permissible in those situations, for example, the way Muhaddithin (scholars of Hadith) have discussed the narrators of Hadith. However, if there is no pressing religious need, and the purpose behind Gheebat is only to do gossip and to enjoy it, then in those situations even telling someone’s true flaws (behind his back) is Haraam (unlawful), and saying something untrue without any effort to find out the truth is ‘Buhtaan’ (slander). The basis of what is said being a lie is not upon making an effort to confirm that it is a lie, but upon not making an effort to find out the truth.”