My personal view is Norton is always the problem. They may be a market leader but I fail entirely to understand why. Given that Norton substitutes their own self signed certificate for the legitimate chain of trust certified ones issued by mail servers so they can do their scanning I can not begin to offer a suggestion on how they could find a problem with something they issued, unless they are looking at the certificate they replace. But I guess you could ask them what the error means and what you should do about it.