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 3, Appendix A, and Appendix C of this publication provide detailed information about hazardous materials that are permitted to be mailed and the conditions that apply.
Chapters 4 and 5 of this publication provide information about restricted matter and perishable matter that either is nonmailable or that may be mailable under specified conditions, as applicable.
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.
Chapter 7 contains information regarding mailability of hazardous materials, restricted matter, and perishable matter in domestic mail via air transportation.
Regardless of content, a mailpiece bearing only postage stamps as the postage payment method and weighing more than 10 ounces or measuring more than one-half inch thick may not be deposited into a collection box, Postal Service lobby drop, Postal Service dock, customer mailbox, or other unattended location. A city, rural, or highway contract letter carrier may not pick up these mailpieces for delivery, either from an individual or through Pickup on Demand 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.