WordPress does not send notifications at all. VPS from reg.ru. What could be?

Question

WordPress

There is a WordPress site. Hosted on VPS by reg.ru. Doesn't send notifications/letters at all. Installed contact form 7 - does not send (writes that it was successfully sent but nothing comes to the mail), even a simple test for sending notifications through "Forgot password" does not send, although again it says that the letter has been sent.
There are a couple more sites on the same VPS, they leave without problems, I don’t know where else to dig.
I read about the option that you need to register a box on the hosting - registered, it's useless.
Letters do not go to any mailbox, tested on @mail, @gmail, yandex .
I do not want to use SMTP, since it leaves other sites, it means that it should also leave this one.
What could be the reason?
I forgot to add, the domain is in the .РФ zone, can the Cyrillic alphabet be the reason?

Answer 1

What is the result of executing the wp_mail function?

Answer 2

Most likely, the function responsible for sending mail using PHP (mail) is turned off in the VPS settings. Are other sites also on WP and don't use SMTP? Be that as it may, always use SMTP. Hostings cut off sending letters via "mail" without warning the user, and spam filters do not let such letters through.