31 C.F.R. § 16.8 – Service of complaint.

(a) Service of a complaint must be made by a certified or registered mail or by delivery in any manner authorized by Rule 4(d) of the Federal Rules of Civil Procedure.
(b) Proof of service, stating the name and address of the person on whom the complaint was served, and the manner and date of service, may be made by—
1. (1) Affidavit of the individual making service;
2. (2) An acknowledged United States Postal Service return receipt card; or
3. (3) Written acknowledgement of the defendant or his representative.