How do Mdaemon and mail work on the mtu-net.ru domain?

Question

Mail server

Help me figure out how the mdaemon mail server and mail in the @mtu-net.ru domain can work. My mosk refuses to understand this.
As far as I understand, the email server can work if it is located on the ip of the registered domain. And, accordingly, letters from other servers are sent to the ip address that the DNS server will say. And here it turns out, as if the server performs the function of a client and picks up mail from mtu-net.ru. But how can he do it?