How to remove parasitic website domains?

Question

Nginx

The other day, Yandex Metrica began to send messages that new domains appeared in the search for which the site is available.
The funniest and dumbest thing is that they look like smtp.sitename.ru and mail.sitename.ru, although I didn’t register them anywhere and didn’t use them in configs. And yes, the site opens by these names, albeit with an ssl certificate error.
How to overcome this fail?
The site runs on nginx, php-fpm
OS Ubuntu
Hosting - free VM from Oracle

Answer 1

Domains are registered not in configs, but at the registrar in the DNS zone