It is illegal to send any article through the US Mail whose composition or material which may kill or injure another person, obstruct mail service or damage property. Harmful matter includes, but is not limited to:
All kinds of poison or matter containing poison.
All snakes, turtles, and spiders, and all poisonous animals, with the exception of live scorpions which are to be used for purposes of medical research or for the manufacture of antivenom.
All disease germs or scabs.
All explosives, flammable material, dangerous machines, and mechanical, chemical, or other devices or compositions which may catch fire or explode; and
Other matter which is likely to destroy, deface, or otherwise damage mail or postal equipment, or which is likely to harm postal employees.
USPS Zip Code Directory, v.2, p.9-11.