California Code, Code of Civil Procedure - CCP § 415.20

(a) In lieu of personal delivery of a copy of the summons and complaint to the person to be served as specified in Section 416.10, 416.20, 416.30, 416.40, or 416.50, a summons may be served by leaving a copy of the summons and complaint during usual office hours in his or her office or, if no physical address is known, at his or her usual mailing address, other than a United States Postal Service post office box, with the person who is apparently in charge thereof, and by thereafter mailing a copy of the summons and complaint by first-class mail, postage prepaid to the person to be served at the place where a copy of the summons and complaint were left. When service is effected by leaving a copy of the summons and complaint at a mailing address, it shall be left with a person at least 18 years of age, who shall be informed of the contents thereof. Service of a summons in this manner is deemed complete on the 10th day after the mailing.

(b) If a copy of the summons and complaint cannot with reasonable diligence be personally delivered to the person to be served, as specified in Section 416.60, 416.70, 416.80, or 416.90, a summons may be served by leaving a copy of the summons and complaint at the person's dwelling house, usual place of abode, usual place of business, or usual mailing address other than a United States Postal Service post office box, in the presence of a competent member of the household or a person apparently in charge of his or her office, place of business, or usual mailing address other than a United States Postal Service post office box, at least 18 years of age, who shall be informed of the contents thereof, and by thereafter mailing a copy of the summons and of the complaint by first-class mail, postage prepaid to the person to be served at the place where a copy of the summons and complaint were left. Service of a summons in this manner is deemed complete on the 10th day after the mailing.

(c) Notwithstanding subdivision (b), if the only address reasonably known for the person to be served is a private mailbox obtained through a commercial mail receiving agency, service of process may be effected on the first delivery attempt by leaving a copy of the summons and complaint with the commercial mail receiving agency in the manner described in subdivision (d) of Section 17538.5 of the Business and Professions Code.