What could be the problem with sending mail?

Question

linux

Hello! The essence of the problem:
There is a mail server that is maintained by yandex - example.ru Now
an additional mail server is configured on the subdomain - mail.example.ru
addresses example.ru, BUT when sending FROM mail.example.ru TO example.ru an error occurs:
May 29 15:52:19 mail2 postfix/smtp[28235]: 4093E20D0E: to=, relay=example.ru[1.2.3.4 ]:25, delay=0.05, delays=0.01/0.02/0.02/0.01, dsn=5.0.0, status=bounced (host example.ru[1.2.3.4] said: 550 relay not permitted (in reply to RCPT TO command ))
Tell me, what could be the problem, could it be related to the curved DNS and MX-record settings? Or do I need to change something in the configs?

Answer 1

The answer googles for one or two