The fact that a derogatory statement was made for a constructive purpose does not permit the listener to accept it as fact. It is permissible to listen to negative information for toeles, a constructive purpose; however, it is not permissible to believe such information. One may act upon such information on the possibility that it might be true.
It is for this reason that the first precondition for speaking constructively (Day 50) is that one have firsthand knowledge of the negative information he is conveying. Since one cannot believe derogatory information as fact, he cannot present it to others as such. In situations where it is permissible to relate secondhand information one is required to say that he heard it from others and could not be sure of it’s accuracy (see Day 71).