But once you send it, it's a matter of the recipient's discretion, not forum rules, what happens from there. This is the normal understanding and you'd better know this before you hit "send."
Sorry but I cannot let you lawyer this. Private communications are private. The rule at FR clearly says that, logic clearly says that and I think ethics clearly says that also. If one wants to disclose something from a private communication one should ask the person for permission to do so first.