A bit of back and forth discourse is fine and expected, but if you keep pushing someone who has their own legal dept they're eventually going to wander over to the coffee machine and have a chat about it with them, then they're going to take a look and it becomes their problem.
So the number one rule would be don't even breath the word "legal" unless you want to invoke them. This particular response is just a letter telling them to back off and it's why you have a legal dept, so they can argue with each other. This one has just found it's way into the open.
There is a understandable perspective that says CAs shouldn't be burdened with legal risk in their discussions, but that's contrary to the fact these guys are commercial entities protecting their interests, so you don't get it both ways unless all your CAs are non-commercial, and even then that would only extend so far.