5 C.F.R. § 1631.42 – Service.

(a) Return of service. Each subpoena shall be accompanied by a Return of Service certificate stating the date and manner of service and the names of the persons served.
(b) Methods of service. Subpoenas shall be served by one of the following methods:
1. (1) Certified or registered mail, return receipt requested to the principal place of business or the last known residential address of the subpoenaed party.
2. (2) Fax or electronic transmission to the subpoenaed party or the subpoenaed party's counsel, provided the subpoenaed party gives prior approval.
3. (3) Personal delivery at the principal place of business or residence of the subpoenaed party during normal business hours.