Rules of Thumb
Just because something can be mailed domestically does not mean it can be mailed internationally. If you have a question about what you are mailing to another country, consult the International Mailing Manual or contact the country's embassy. If you have questions about domestic mail, contact your local post office. Any questionable items may be subject to a ruling regarding its suitability for mailing. It is the sole responsibility of the mailer to comply with federal, state and local ordinances, as well as USPS regulations.
Hazardous materials include anything that is a significant risk to public health, safety or property and are generally prohibited from air and surface transportation as well as international mail and APO/FPO mail by the USPS unless they are considered ORM-D materials. The Department of Transportation determines which materials are considered hazardous according to nine categories: explosives; gases; flammable and combustible liquids; flammable solids; toxic substances and infectious substances; oxidizing substances and organic peroxide; radioactive material; liquid and solid corrosives; and miscellaneous materials.
Restricted matter is anything that has been prohibited for reasons other than public safety. These substances and materials include intoxicating liquors; firearms; knives and sharp instruments; liquids, powders and odor-producing materials; motor vehicle master keys and locksmithing devices; controlled substances and drugs; and anything used for animal fighting ventures. These substances are generally prohibited by the USPS for mailing unless the sender and recipient are specially licensed or are members of the military or law enforcement, among other exceptions.
Perishable matter includes items that can deteriorate over the course of the mailing and could pose a health risk, cause a mess or be a nuisance. It is at the discretion of the mailer as to whether their items fall within this description, and it is up to the mailer to arrange schedules to ensure that their items will arrive before they become a risk or nuisance. While there are exceptions, live animals are considered prohibited perishable material.