S.D. Codified Laws § 21-18-11
Such garnishee summons, affidavit, and garnishment disclosure may be served by certified mail, return receipt requested, or personally by the sheriff of the county where any garnishee or defendant may be found, or by any other person not a party to the action. The person subscribing such garnishee summons may, at his option, by an endorsement thereon fix a time for the service thereof, and the service shall then be made accordingly. The service shall be made and the same returned, with proof of the service, to the person whose name is subscribed thereto, with reasonable diligence. If, after reasonable diligence, service by certified mail or personal service by the sheriff is unsuccessful within the time period provided for in § 21-18-10, the plaintiff may file an affidavit stating that fact and may serve such garnishee summons, affidavit, and garnishment disclosure on the defendant by mailing a copy of the documents, together with a copy to the garnishee, to the defendant's last known address by registered mail. The garnishee may proceed by complying with either § 21-18-32 or 21-18-33 and, by such compliance, is exonerated from any further liability to any party to the garnishment.
SDCL 21-18-11