45 CFR § 2554.12 - How will the complaint be served?
(a) The complaint must be served on individual defendants directly, a partnership through a general partner, and on corporations or on unincorporated associations through an executive officer or a director, except that service also may be made on any person authorized by appointment or by law to receive process for the defendant.
(b) The complaint may be served either by:
(1) Registered or certified mail (return receipt requested) addressed to the defendant at his or her residence, usual dwelling place, principal office or place of business; or by
(2) Personal delivery by anyone 18 years of age or older.
(c) The date of service is the date of personal delivery or, in the case of service by registered or certified mail, the date of postmark.
(d) Proof of service -
(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 receipt by the defendant or a representative, will serve as proof of service.