Rules on Civil Procedure
Section 7. Service by mail. - Service by registered mail shall be made by depositing the copy in the post office in a sealed envelope, plainly addressed to the party or his counsel at his office, if known, otherwise at his residence, if known, with postage fully prepaid, and with instructions to the postmaster to return the mail to the sender after ten (10) days if undelivered. If no registry service is available in the locality of either the senders or the addressee, service may be done by ordinary mail. (5a; Bar Matter No. 803, 17 February 1998)