(a) Each order, notice, or other document required to be served under this part shall be served personally or by registered or certified mail, except as otherwise provided herein.(b) Service upon a person's duly authorized representative constitutes service upon that person.(c) Service by registered or certified mail is complete upon mailing. An official United States Postal Service receipt from the registered or certified mailing constitutes prima facie evidence of service.(d) Service of requests for admission and motions may be made by first-class mail, postage prepaid.(e) Each pleading must be accompanied by a certificate of service specifying how and when service was made.42 FR 56742, Oct. 28, 1977, as amended at 54 FR 42906, Oct. 18, 1989