Determinations, notice of hearing, complaints, respondents' answers, and division decisions, findings of fact and orders shall be served by personal service or registered or certified mail, or ordinary, first class mail. However, where a nonresident person or foreign corporation is charged with violating any provision of the law by virtue of the provisions of section 298-a thereof, the complaint and notice of hearing shall be served only by personal service or by registered mail, return receipt requested, directed to such person or corporation at the last known place of residence or business.