1. Actual malice (in legal terms this means you know it's false and you did it to hurt someone) 2. The statement has the possibility to harm someone's reputation (lying about what some dude ate for breakfast isn't going to hold anyone culpable unless you're saying they ate human infants or something) 3. The statement can be interpreted as fact (saying that someone is always on Santa's naughty list also isn't going to have culpability) 4. The statement actually caused harm to the target.
This is why the Tucker Carlson "no reasonable person would believe what he says" argument got famous. Alex Jones tried it as well but it didn't work out for him.