Add subforum for the FQDN

Hi all, I have a domain (example: mysite.xyz) and the FQDN (hostname -f) of my VPS is mail.mysite.xyz

I've tried to create a mail subdomain of mysite.xyz, so mail.mysite.xyz... but when I put some html or php files into public_html folder and I try to access them via browser, I get the error "404 Not found".

If I write into the browser mail.mysite.xyz, I get the apache default page instead of the index.html file I've placed into public_html folder (/home/mysite.xyz/domains/mail.mysite.xyz/public_html).

Is this a restriction? I mean, is it not possible to have a subdomain that is equal to the FQDN?

Thank you in advance for the answer.

Cheers.