29 CFR §2570.83

1. (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.
2. (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.