Creating a subdomain: having problems with the mail server?

Question

Domain Name System

Hello!
I had an example.ru domain with Yandex mail servers .
I recently bought hosting from another company .. without a domain name .
I decided to create a subdomain admin.example.ru specifying the IP address of the new hosting.
To do this, I went to the domain control panel (reg.ru) and created a record like A:admin.example.ru and an IP address on the new hosting.
Saved. By the name of the site admin.example.ru, I go to a new site. But
a letter arrived informing that the Yandex mail service no longer works for the example.ru domain.
you need to add MX records and stuff like that
What could be the issue here? After all, I just created the admin.example.ru subdomain, and did not touch any mail server .
By the way, the domain on reg.ru

Answer 1

I didn't understand anything :) example.ru or example.su ?

Answer 2

1) on reg.ru for the main domain example.com, write dns: dns1.yandex.net, dns2.yandex.net
2) add the main domain (*, www) in Yandex traffic rules
3) add the admin subdomain there
Everything should work, I do this with all my domains.