Non-mailable is any mail or material that is generally prohibited for mailing. When these materials are found in the mail stream, they will be held by the postal inspection service. These items include hazardous materials, firearms, explosives, switchable knives, incendiary devices, motor vehicle master keys, controlled substances, advertisements camouflaged as invoices, and perishable material that may go bad before delivery.
Here are things that the postal service must refuse to carry:
• Items that are prohibited by law. For example, fraudulent, illegal or obscene materials.
• Any item that breaches an Act of Parliament
• Products that do not meet marking requirements or certain characteristics
• Any product or substance that could:
• Damage postal equipment or any other thing
• Injure and cause harm to people handling the mail
• Cause entrapment of other materials
• Sexually explicit material unless they’re sent in an opaque wrapping and labeled “Adult Material” or words along those lines.