How to solve the problem of mail circulation between 2 domains?

Question

Email

There are 2 domains purchased from 1 and the same provider (right in 1 contract). Both of them work normally both for reception and for transmission. But there is no mail exchange between each other :(. Spam filters are turned off, the logs are empty. The letter just leaves and that's it.
What could be the problem?

Answer 1

What postal service do you use?
I would like you to set the logging level to the maximum and post the logs here. from both servers.
Also, check the routing. The DNS information for both servers is probably in a table on your DNS server. But mail servers can be located in different VLANs and not have access to each other.
Also, try to do: nslookup -type=mx <other_mail_server> and post the result here, along with the logs.
You may also need a detailed description of your network config in terms of these servers, DNS, routers, switches. Including configs of this iron (in words it is enough) with VLANs, Trunks and other things.
In short, more details are needed.

Answer 2

Vasily is great, but until pippu learns to communicate in technical language: "here is the log, here is the expectation, here is the environment / settings" all advice is like peas on the wall