Under 18 U.S.C. 1716, all matter that is outwardly or of its own force dangerous or injurious to life, health, or property is nonmailable. The knowing deposit of such matter in the mail is a crime punishable by fine, imprisonment, and/or other penalty.
For reasons of safety, most hazardous materials are nonmailable. However, some hazardous materials and otherwise restricted matter, or perishable matter are permitted to be mailed when the requirements in this publication are fully met.
Chapter 6 specifies the mailing conditions that apply to hazardous materials, restricted matter, and perishable matter in international mail, including hazardous materials for delivery to overseas military and diplomatic Post Office (APO/FPO/DPO) addresses.
Regardless of content, a mailpiece weighing more than 13 ounces bearing only postage stamps as postage may not be deposited into a collection box, Postal Service lobby drop, Postal Service dock, customer mailbox, or other unattended location. These mailpieces are also precluded from pickup service. The sender must present such items to an employee at a retail service counter in a Postal Service facility. Improperly presented items will be returned to the sender for proper entry and acceptance.