It’s an offence if you use the mail for the delivery of:

• Articles that are obscene, indecent, immoral, or scurrilous
• Information relating to bookmaking, pool-setting, betting, or wagering
• Articles relating to unlawful lottery schemes
• Articles relating to schemes to deceive or defraud the public
• Articles or special messages sent to any person intending to obtain money under false pretenses

It’s also an offence to make false representations on any customs document.