I recently had some inquiries about addresses in specific countries from WorldVu's customers and was checking the UPU's most recent changes to their Postal addressing systems in member countries. The UPU's information is received directly from or verified by postal or government authorities in the specific country. I use those sources, too. I also use private businesses, embassies, and individuals around the world.
I would very much like to report that everyone in particular countries agrees on how to write postal address there; I cannot do so. Ask someone – anyone – from Germany or Japan, the U.S. or the U.K., South Korea or Sweden, Canada or China how to write an address and they can tell you the format. (In Japan they might well give you 2 or 3 variations that are all equally correct.) These and 50 or so other countries have postal addressing standards that are very specific.
Then there is the rest of the world, which also happens to be the majority of the world both in the number of countries and in population, although not in volume of mail. In the rest of the world, the "correct" address format may depend on who one asks. The postal code may officially exist but it may be used rarely. When it is used, its format may not be standard – does it have an internal space or not, a hyphen or not? The same goes for whether a province or province abbreviation is used – and what the abbreviations are. There are no official addresses or no official addresses outside of main business and residential districts in cities and larger towns in many countries.
The problem for those of us who mail internationally is that the "official" address standard where they are defined is not used, is not widely known, is ignored in many countries. In reality, the address standard from the government or the postal authorities does not describe the addresses that are in day-to-day use. By WorldVu's count, the province or postal code is often used in addresses in about 35% of the countries that officially use a province or a postal code.
Where address standards exist and are used by residents, using addresses that meet national standards will insure that your mail or shipment is delivered and delivered more quickly. The item will create a better impression on the recipient, looking correct and professional. Where address standards are not used by residents or don't exist, use the address the resident provides. International mail that does not meet national (domestic) requirements is usually delivered but the delivery may be slow – sometimes very slow. If you are mailing or shipping merchandise, try to get confirmation of delivery or check with the recipient after a suitable time if possible.
How do you know what countries have standards and which ones don't? Experience and research or by subscribing to WorldVu's Guide to Worldwide Postal-Code and Address Formats.