45 CFR §1174.8

1. (a)The reviewing official must serve the complaint on an individual defendant directly, on a partnership through a general partner, and on a corporation or an unincorporated association through an executive officer or a director, except that the reviewing official may also make service on any person authorized by appointment or by law to receive process for the defendant.
2. (b)The reviewing official may serve the complaint either by:
   1. (1)Registered or certified mail; or
   2. (2)Personal delivery by anyone eighteen years of age or older.
3. (c)The date of service is the date of personal delivery or, in the case of service by registered or certified mail, the date of postmark.
4. (d)When the reviewing official serves the complaint, he or she should also serve the defendant with a copy of this part and 31 U.S.C. 3801-3812.