When you address an envelope in Japan bound for Japan, you write vertically. Write the name of the business or person you want to reach in the middle of the front of the envelope, the address in the upper right corner under the zip code boxes. (The 7 small red boxes at the top of envelopes is meant for the recipient's zip code.) The first line of the address will be far right, and any extra lines will go to the left of the first line.
For a letter bound for a business, you will write the name of the business and then add 御中 (onchuu) to the end.
For a letter bound for a person, you will write the person's name in the order family name then first name, and add 様 (sama, Mr./Ms.).
I want to note one mistake I made before
I wrote 行 instead of 御中
Regardless, that was what I was instructed by the JLPT office
Although I am unsure as to why, it should send
Then, still on the back of the envelope, write the return address vertically in the top right corner. it doesn't matter if it is a business or person's address in how you write this part.
Here is a breakdown:
(prefecture) (city) (town or ward) (house #)
In an example case:
山口県 (yamaguchi-ken, Yamaguchi Prefecture)
岩国市 (iwakuni-shi, Iwakuni City)
麻里布町 (marifu-machi, Marifu Town)
２丁目１−２０ (2 choume 1-20)
The address is actually for a Mister Donut, so the result would be:
Mister Donut 御中
740-0018
山口県岩国市麻里布町２丁目１−２０
Writing a return address follows the same pattern, except you don't add any honorific after your own name. Even in Japan if you do not have a Japanese name, it is best to write vertically but in English lettering.
When addressing international mail, always put the name of the country under both recipient and return addresses in all capital letters.
There are still some variations you may want to be aware of:
1. 県 (-ken) is the indication for prefecture except in the cases of Tokyo and Osaka. Tokyo Prefecture is 東京都 (toukyou-to), and Osaka Prefecture is 大阪府 (oosaka-fu).
2. Not all addresses are alike so some addresses may show a city（市, -shi）and then a town（町, -machi/-chou）, but others may show a city and a ward（区, -ku）.
3. There are two ways to express a house number. One uses 丁目 (choume, chome -when written in English) after the first number like ２丁目１−２０. The other option is ２−１−２０.
4. There is a symbol in Japan that always indicates a zip code, I will attach the image below. On most envelopes there is a space where you should write the zip, so you will not need to write it. On envelopes that don't have it, adding it horizontally above the address lines with the symbol is best.
this symbol does not need to be written in red on envelopes
it also indicates post offices on signage in Japan
Please feel free to ask me if you have further questions.
