Why don't letters from Yandex and Ramler reach the mail server?

Question

Mail server

Good afternoon.
Previously, the company's mail was on the Yandex mail server, but now they set up the server on the iredmail assembly and 2 dns servers, transferred imapsyns mail and had problems receiving mail from yandex .ru and @rambler.ru addresses. Until they switched to our DNS and Yandex DNS was functioning, letters from Yandex came only to the old Yandex server, although there were not even mx records for Yandex. Disabled Yandex DNS and delegated the rights to our DNS and now letters from Yandex do not reach anywhere at all.
What could be? The logs are empty, where to dig?

Answer 1

Check answers from dns servers, you can use 2whois
ask a question on MX to your dns, google dns, and yandex dns, the answer should be the same everywhere - MX looks at your server.
Then look at the logs of your MTA.