Whenever the superintendent of an institution withholds from mailing letters written by inmates of such institution, the superintendent shall forward such letters to the secretary of corrections or the secretary's designee for study and the inmate shall be forthwith notified that such letter has been withheld from mailing and the reason for so doing. Letters forwarded to the secretary for study shall either be mailed within seven days to the addressee or, if deemed objectionable by the secretary, retained in a separate file for two years and then destroyed.