It's complicated, but this is explicitly not recognized as a defense by courts. If Alice tells me "Bob says that Charlie started the fire", whether or not that

It's complicated, but this is explicitly not recognized as a defense by courts. If Alice tells me "Bob says that Charlie started the fire", whether or not that statement is defamatory depends on whether or not Charlie actually started the fire, not on what Bob actually said. The more likely legal defense is Section 230 of the Communications Decency Act, which shields online service providers for liability for hosting online content from third parties.