Perhaps you should do some of that Law Research and find a cite. As I already have. If you are in a law office, pick up a Hornbook and see the black letter law on the matter. Since the law has been that way from the turn of the century, I don't see any modern cases easily accessible. With your skills, I'm sure you can come up with something.
The company is not the person. An employee of the company is not the company. Say I go to a person and call them a stinkyhead and did not know anyone else is around. But, just as I said it the parish priest entered the room and he hates stinkyheads and excommunicates the person. Did I publish the defamatory term? (Assume it is false and defamatory etc. We're talking about publication.) Say I put an unsealed envelope on stinkyheads desk and someone else comes in and reads it, was it published? Let's say the OP calls the corporation stinkyheads and the employee reading the letter shows it to the press who publishes it without authorization. Who are the publishers, just the press and employee or also the OP?
Now, I agree the priest issue is one that is closer as accidental or non-negligent publication is not one that generally leads to liability. There, words spoken with no reason to suppose that anyone but the plaintiff would overhear them, or a sealed letter sent to the plaintiff himself which is unexpectedly opened and read by another would not be either. Note "unexpectedly". The person the letter would be sent to in this case IS NOT THE CORPORATION. He might be an employee of the corp, or an agent of the corp, but it is not the corp. There is publication.
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The lack of harm is a separate question and I have already mentioned that repeatedly. However, say there are damages. Was the letter to the company published?

