Receiving COE while in Japan

Hello I am Luka from Japan, currently working in Japan from last 2 years.
I applied for spouse visa in December for my wife. I have been working in Japan with valid working visa which is until 2020.
Today I got unattended item notice ( from JP post ) . I wasnt at home.
Its merely been a month , and I got some notice from immigration .
I want to know will it may contain rejection letter or COE ?
As there was no one at home to receive the mail, it is def IMPORTANT matters. But I would like to know from those who obtained their COE for Spouse visa, how did they receive it?
Did they receive from same postal mail , or will you get that small card , where you have to take and go immigration again ?
Thank you

COEs are sent by postal mail. However, a COE is used by those applying for a visa outside of Japan. In your case, you should - instead - apply for a "change of residence status" at an immigration bureau inside of Japan. At least this is my understanding.

The question is a bit confusing but I understood that, Luka, you are working in Japan on a work visa, and you applied for “dependent” visa for your wife? And as the applicant, Luka should receive the CoE for the wife (who is outside Japan now?)
For “change of resident status” or renewal, you receive a postcard. For others, it would come in an envelope, and I believe it needs to be handed in person.

since you were absent, you got 不在通知.
the easiest ways to collect the letter:
1. ask the help by your Japanese friend to call post office. (reply phone is a bit complicated. need complete understanding of the language.)
or
2. go to 集配郵便局　with that slip and your ID which shows your address.

if you don't do that, the letter will be returned to the sender after (about) 1 week.

Simple - take the notification to the post office indicated and get the item. Then you will find out what the mail is. Picking up registered mail is a simple/common thing.