In representing a client, a lawyer should not communicate about the subject of the representation with a person the lawyer knows to be represented by another lawyer in the matter, unless the lawyer has the consent of the other lawyer or is authorized to do so by law or a court order. The lawyer must actually know the client is represented, but he cannot close his eyes to the obvious. Courts will enforce this rule even if the lawyer has only circumstantial knowledge of the representation. The parties are not limited to the parties in the litigation. Commonly, courts preclude evidence and "fruit of the poinsonous tree" obtained through ex parte contact.
See: ABA Model Rule of Professional Conduct 4.2