ilker
Just to double check, send an email to a temporary email like the ones in the test on the following site:
https://appmaildev.com/en/dkim
This can test to make sure the email itself is set up properly. Flarum will sometimes say its set up, usually SSL or TLS when the email itself wont send properly that way. I'm having issues myself with only STARTTLS working due to non working DKIM/DMARC/SPF and emails themselves only working to non-large providers. So you could be in a similar boat. Worth a look, hope it helps/clarifies what the issue is.