Mailing inappropriate articles may present a serious threat to the safety of the article as well as the safety of members of the postal service. Dangerous or inappropriate articles include: (a) articles which, by their nature or their packing, may expose officials to danger, or soil or damage correspondence; (b) opium, morphine, cocaine and other narcotics; (c) articles of which the importation or the circulation is prohibited in the country of destination; (d) living animals; (e) explosives, inflammable or dangerous substances; (f) obscene or immoral articles.