Straight from the USPS Site:
Nonmailable and Restricted Articles and Substances Generally
8.6 Hazardous Materials
Harmful matter also includes regulated hazardous materials as defined in 10.0 that are likely to harm USPS employees or to destroy, deface, or otherwise damage mail or postal equipment. This includes materials such as caustic poisons (acids and alkalies), oxidizers, or highly flammable liquids, gases, or solids; or materials that are likely, under conditions incident to transportation, to cause fires through friction, absorption of moisture, or spontaneous chemical changes or from retained heat from manufacturing or processing, including explosives or containers previously used for shipping high explosives with a liquid ingredient (such as dynamite), ammunition, fireworks, radioactive materials, matches, or articles emitting obnoxious odors.
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