Mail can be separated into two types - domestic and international. Domestic mail is exchanged between people in the same country, and international mail is exchanged between people in different countries. International mail can only be delivered with the cooperation of two or more countries.

When you send a letter or a package, it is taken to a post office. At the post office, all of the mail has to be sorted so that it can go to the correct destination. Much of this process is now handled with modern machinery. The mail is first separated into piles of letters and packages, sorted by class, and the stamps are canceled.  It is then sorted into groups of mail heading to the same destination or direction.

Low class mail will be sent by the cheapest transportation, such as ground or sea. This low class mail will sit around for long periods of time so that enough of it can be collected to ship in bulk loads. High class mail will be delivered faster, possibly by airplane, and will not wait as long between stops.

Along its journey, each piece of mail can be checked for security hazards in a variety of ways.  It could be as simple as a check for any odd-shaped packages, or perhaps it could pass through an X-Ray machine.  Any item which poses a possible threat can be confiscated at the post office.

There are many modes of transportation used for delivering mail.  Some mail will be delivered by ground or sea transportation, and airmail will be sent to the airport. At the airport, bags of airmail are loaded on to commercial flights headed to the same destination as the mail. When the flight reaches another country, it becomes the sole responsibility of that country to ensure that the mail is delivered, and it is no longer in the hands of the country from which it was sent.

The majority of all mail will travel through several post offices before it reaches its final destination. For domestic mail, many countries use a postal code, a series of numbers representing specific districts within the country. These codes help to make the sorting process more mechanical and efficient.