45 CFR § 1149.16
What constitutes proof of service?
April 14, 2021
(a) Proof of service is established by the following:
(1) When service is made by registered or certified mail, the return postal receipt will serve as proof of service.
(2) When service is made by personal delivery, an affidavit of the individual serving the complaint, or written acknowledgment of your receipt or of receipt by a representative, will serve as proof of service.
(b) When served with the complaint, the serving party must also serve you with a copy of this part and 31 U.S.C. 3801-3812.