How to properly configure NS DNS records for MTA (Postfix)?

Question

System administration

Hello!
Trying to get Postfix up on Ubuntu 20.04. Faced with a misunderstanding of the NS DNS records settings (I immediately apologize if I write something incorrectly).
There is a domain, the main NS record eample.ru type A points to one IP (uncontrolled server, you cannot change the IP). There is a subdomain api.example.ru (record type A points to a controlled server, on which I am trying to raise Postfix). What records should be specified in NS so that the controlled server accepts example.ru mail?
mail.example.ru (CNAME) = mail.example.ru?
do I need to specify MX records on the IP of the controlled server?
Help me figure it out please :)

Answer 1

NS is nameserver
And for the mail server there should be an MX record - mail exchanger

Answer 2

The DNS server responsible for the root zone contains an NS record that points to the DNS server (the IP address of the DNS server) that contains the ru. More precisely, one NS record for each DNS server in this zone.
The DNS server responsible for the zone rucontains an NS record that points to the DNS server that contains the zone eample.ru.
And also downward.
However, any server in the chain can intercept the subdomain/subzone/subzone.
Those. A record of any type for a DNS zone api.example.rucan be located on both the root DNS server, and the DNS server of the zone ru, and the DNS server of the zone eample.ru.
Pay attention - here we are talking about DNS servers. They may or may not be responsible for the mail, as you do.
For mail servers, MX records are written indicating the "metric" - first, the mail server that sends the letter will try to send mail to the mail server with the lowest metric, and if it fails, it will try in ascending metrics.
Postal addresses are called strictly by name, the IP address is written separately - sometimes in the same zone, sometimes somewhere else.
And also pay attention to the fact that short or full (FQDN) names can be recorded in DNS zones. Short names are written as is and are automatically completed with the zone domain. Full names must end with a dot. IP addresses are written without a dot at the end, because it is understood that they are only complete.