When a letter is sent, via email, from a lawyer on behalf of another party, threatening action due to accused defamation, are they only legally allowed to include the two parties involved in the email? Is it illegal for them to include other parties not involved, or is it just frowned upon?
It is unusual for a law firm threatening defamation action on behalf of their client to send their threatening letter to more than the party they intend to sue, but there is nothing to stop them from doing so.
Without knowing more details it is difficult to comment, other than to say that the purpose for so doing might be to show the third party that they believe their client has been defamed and they are not going to put up with it.
Please remember however, that one of the defences to any defamation action is truth.