Many companies shipping goods that may be problematic are aware that there may be restrictions on their goods and work to meet the legal requirements for their shipment – authorizations and licenses to import or export the goods legally. Those mailers or shippers new to international sales may not be aware of the requirements.
In general, any article that cannot be sent in domestic mail cannot be mailed internationally. Some goods such as dangerous, infectious or radioactive material are subject to restrictions of movement due to their very nature, irrespective of their origin or destination. When it is possible to move these items by mail, they must be accompanied by various documents – licenses, certificates, or other authorizations to import or export – that must be presented to the customs or other agency as required by that agency and the postal authorities.
Restrictions on certain categories are very clear and documented, such as Category A infectious materials or narcotics, while other categories allow room for judgment. Others are more subjective and need to be determined before mailing or shipping. (Of course, most items are not subject to restrictions and transit the postal system without a problem.)
Limitations and prohibitions on the contents of items mailed are subject to a great deal of variation among countries. Individual countries can prohibit certain items or classes of items that otherwise could be transmitted by mail. Canada prohibits the importation of "oleomargarine and other butter substitutes" and the U.S.A. prohibits the importation of tobacco. Other categories of goods are also prohibited by specific countries. Jordan prohibits table salt and the United Kingdom prohibits "horror comics and matrices".
Individual countries also prohibit sending all or particular items to persons or organizations that they define as engaging in illegal activities, such as terrorism, narcotics trafficking, or fraud. There may also be prohibitions on trade with certain countries. These restrictions are specific to each country. Many countries with substantial volumes of international trade frequently provide this information on government web sites.
In all cases, the customs in the destination country or other government agencies in the country of origin or destination decide. Erring on the side of caution may prevent the confiscation of the items, fines, legal actions, and a loss of time in appealing a decision by a government agency in the country of origin or the country of destination.
The UPU provides more details about prohibited items on their web site. The USPS provides categories of articles prohibited by individual countries. Australia Post has specifics on international delivery and restrictions can be found and La Poste (France) has information on specific countries.
If you believe your mail may contain material that is subject to regulation, check before sending and avoid the possible legal problems, fines and confiscation.