Yes. Your husband's mistress could sue you in civil court for harassment and claim that the harassment led to mental anguish. Even if she is in the wrong for having an affair, you cannot legally harass her through unwanted contact. In all potential legal matters, you should seek the advice of a licensed attorney. You can read more about suing for harassment here: http://canon-mcmillan.patch.com/articles/ask-the-attorney-suing-for-harassment.