5 CFR § 1201.84 - Proof of service.
The person who has served the subpoena must certify that he or she did so:
(a) By delivering it to the witness in person,
(b) By registered or certified mail, or
(c) By delivering the subpoena to a responsible person (named in the document certifying the delivery) at the residence or place of business (as appropriate) of the person for whom the subpoena was intended.