27 C.F.R. § 771.26
All orders, notices, motions, and other formal documents required to be served under the regulations in this part may be served by mailing a signed, original copy thereof to the designated representative of the applicant, licensee, or permittee by certified mail, with request for return receipt card, at the representative's business address, by personal service, or as otherwise agreed to by the parties. If the applicant, licensee, or permittee has not yet designated a representative, all orders, notices, motions, and other formal documents required to be served under the regulations in this part may be served by mailing a signed, original copy thereof to the applicant, licensee, or permittee at the address stated on his application, license, or permit, or at his last known address, or by delivery of such original copy to the applicant, licensee, or permittee personally, or in the case of a corporation, partnership, or other unincorporated association, by delivering the same to an officer, or manager, or general agent thereof, or to its attorney of record. Such personal service may be made by any employee of the Department of Justice designated by the Attorney General or by any employee of ATF. A certificate of mailing and the return receipt card, or certificate of service signed by the person making such service, shall be filed as a part of the record.
27 C.F.R. §771.26