When looking up "libel" in a dictionary, it says
a published false statement that is damaging to a person's reputation; a written defamation.
I am pretty sure there are laws for almost any juristiction covering this.
We had a case posted over at another SE site that made me curious about one specific aspect.
Since I don't know enough details of that specific case, I will make one up here to satisfy my own curiosity and provide all the details neccessary. Since I am not a lawyer, I will also post my assumptions about the laws that I think I know, feel free to jump in and correct me on that if any of those are wrong.
The laborer Larry is very unhappy with his boss Bob. Larry accuses Bob of something that is untrue and damaging to Bob's reputation.
My assumption is that if Larry does that in a private conversation, letter mailed to Bob's private address or even letter to Bob's work address, then there is no third party, hence no "publishing". Bob can read the letter and he can take actions based on it, maybe even fire Larry, but it is no libel. If Bob decides to show it to someone, the "publishing" is Bob's doing and Larry cannot be held accountable for that.
If Larry includes third parties (lets say he sends a copy to HR and the company CEO Charlie) then it may be libel, since it was "published" by Larry.
What if the letter was send to Bob only, but it is Bob's duty to give it to third parties?
Lets say the letter is actually Larry's resignation. It says something defamatory and ends with " ... and this is why I quit. I will serve my notice, my last day will be $date".
Legally, giving this letter to Bob the boss is just a courtesy. The contract is with the company, the company is represented by HR, the legal minimum action would have been to mail the letter to HR. When Bob gets this letter, Bob is required to give it to HR. Bob can neither decide to ignore it, nor can he do all the contract resolution stuff himself.
HR, upon receiving the contract termination document with the accusations, feels it is their duty to bring this to the attention of the CEO Charlie, since they cannot independently verify the accusations and their corporate handbook says to bring up any problematic behaviour with the accused person's line manager.
So now Larry wrote a defamatory statement about Bob. Larry did not directly send it to anyone but Bob. But Bob had no choice than to show it to HR and HR had no other choice than to show it to Charlie. Given that those mechanism aren't exactly secret, Larry should have known about them. Whether he did when writing the letter, nobody can say.
Would writing something to a person seemingly in private, when it should be known that this person is required to "publish" it themselves considered libel?
I am most interested in my own juristiction Germany, but the original inspiration for my made up case came from the United Kingdom. As always, other answers are welcome too, I'm always curious how other justice systems handle something.