A frequent mistake in addressing international mail pieces is an incorrect postal code format.  But, an error here can cause your mail to be greatly delayed or not delivered at all.  It can be difficult to know if a postal code is correct with so many different formats and the fact that some countries don’t use them at all.  Often, the problem is trying to fit a foreign postal code into a domestic zip + 4 format.  There are many resources available to find out if your destination uses postal codes, or what the proper format is.  One is the Universal Postal Union, which has a section for each country.  You can also contact your international mail specialist, like International Mail Service.