If I say Mr X is, for example, a serial rapist, then I must be able to provide proof of my assertment.
If I cannot, and Mr X loses his job, family and everything, and his quality of life is negatively influenced by what I said, then yes, he can sue me.
On the other hand, if he is a serial rapist, and I can prove it, then he can do nothing.
I know - it was just a joke
But it would be funny trying to prove it in court.... I have witnesses.