Hello 
I couldn't find any info   ( or my search on google is lame )

Currently i want to change the domain name, of my website.
From X to Y. what changes should i do in the config  / database to change the domain ?

@StefanGr
You must update website link and database connection.
you can discuss this with hosting provider.

you can discuss this with hosting provider.

  I did that already.
I wonder  if there is any special setup to do... f.e in wp you need to configure the website name in the config , and in database several places.

In my case, the hosting provider is the same, the database is the same, files are on the same server / path , only the domain name is change.

You should not need to update anything else then config.php.
Anyway you can check t_preference table to be sure.

I changed domain name, config.php and preferences. I receive contact emails, but I don't receive emails about new classifieds. What should I do?

Check Settings > Mail server

It's software bug.

Then report to PHPMailer github.

I receive check email and contact email, but I don't receive emails about new classifieds.

Still can be mail server issue. Try different mail server.
Check error log as well.

I've tried all mail ports, that web hosting company gave me. When I first installed Osclass everything was working fine. But then I changed domain name and I got this issue. Maybe I need to change domain settings in back end? I think old domain settings hidden somewhere. Or maybe I need to somehow clear cache?

No cache.

Had same problem,  t_preference table was missing some data after domain change. Strange but easy fix.

Then you've migrated partially only 

I found in oc_t_admin table that old email address was not changed by Osclass in back office. I changed it manually in phpMyAdmin to new email address. Now everything is working! Please fix this bug in Osclass.