My question involves malpractice by a lawyer in the state of: New Jersey

I have conflicting information and would like some guidance. I have been advised by one attorney that an attorney is legally required to acknowledge and respond to all correspondance that said attorney receives when the correspondence is related to a case in the attorney represents a litigant, regardless of whether the adversary is represented or is pro se.

On the other hand, I have been told by another attorney that neither acknowledgement of correspondence, nor response is required.

So, my question is thus, is a lawyer legally allowed to simply ignore or not respond to correspondence in a case in which that lawyer is representing one of the litigants? Please direct me to the law if there is a such a law, or if it is more a matter of ethical practice and less mandatory requirement, please similarly advise.