The method for effecting service by mail is set forth in Code of Civil Procedure section 415.30. It provides that a summons may be served by mail by depositing it in first class mail, among other things, two copies of the notice and acknowledgment in the form described in subdivision (b) of that section. (Code Civ. Proc., § 415.30, subd. (a).) Service of summons is only complete when written acknowledgment of receipt of summons is signed and returned to the sender. (Id., at subd.Â*(c) & (e).)1 Thus, the efficacy of service by mail is expressly based on the execution and return of an acknowledgment of service. (Thierfeldt v. Marin Hosp. Dist. (1973) 35Â*Cal.App.3d 186, 199; Code Civ. Proc., § 415.30, subds. (c) & (e).) “If the party addressed fails to do so, there is no effective service.” (Thierfeldt v. Marin Hosp. Dist., at p. 199, italics added.) If acknowledgment of service is not returned within 20Â*days, the Code of Civil Procedure provides plaintiffs with recourse. (See, e.g., Code Civ. Proc., § 415.30, subd.Â*(d).)2