Following the model put forth by the United Nations, the Department of Transport (DOT) divides hazardous materials into nine different classes, and mailing hazardous materials requires that you obtain a placard and make sure the contents of the package are clearly labelled on the package itself.

These classes are:

Explosives

Gases

Flammable Liquids

Flammable Solids

Oxidizing Agents and Organic Peroxides

Toxic and Infectious Substances

Radioactive Substances

Corrosive Substances

Miscellaneous

Adhesives are, luckily, much easier to ship than many other hazardous materials or dangerous goods, but are still considered dangerous because of their toxicity and their flammability, placing them into classes 3 and 6.