5 C.F.R. § 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,
(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, or
(d) By any other method that is in accordance with applicable State law.