S.D. Codified Laws § 15-9-19

The court or judge must also direct a copy of the summons and complaint to be forthwith sent by first class mail, directed to the person to be served at his post office address unless it appears that the place of residence and post office address of such person is neither known to the party making the application nor can with reasonable diligence be ascertained by him; and also directed to any corporation to be served at its present or last known office or place of business.