29 C.F.R. § 2570.83 – Assessment of civil penalty.

(a) Except as described in §§ 2570.85 and 2570.86 of this part, subsequent to the payment of the applicable recovery amount pursuant to either a settlement agreement or a court order, the Secretary shall serve on the person liable for making such payment a notice of assessment of civil penalty equal to 20 percent of the applicable recovery amount.
(b) Service of such notice shall be made either:
1. (1) By delivering a copy to the person being assessed; if the person is an individual, to the individual; if the person is a partnership, to any partner; if the person is a corporation, association, exchange, or other entity or organization, to any officer of such entity; if the person is an employee benefit plan, to a trustee of such plan; or to any attorney representing any such person;
2. (2) By leaving a copy at the principal office, place of business, or residence of such individual, partner, officer, trustee, or attorney; or
3. (3) By mailing a copy to the last known address of such individual, partner, officer, trustee, or attorney.
  If service is accomplished by certified mail, service is complete upon mailing. If done by regular mail, service is complete upon receipt by the addressee.