How to set up a DNS entry to send mail from a subdomain?

Question

Domain Name System

Hello!
There is: a server with the resource.com domain (ip 77.66.55.44) and the lk.resource.com subdomain (ip 44.55.66.77). An application is running on the lk.resource.com server that sends a registration confirmation letter to users using postfix. If you specify the header From: [email protected], the letter gets into spam. I know that I need to adjust the spf and dkim entry.
What domain should be registered on and how to correctly compose the spf so that the lk.resource.com server (ip 44.55.66.77) can send letters on behalf of the resource.com server (ip 77.66.55.44) and not fall into spam?

Answer 1

The letter leaves from a name (host) of the server. DNS is the same as for the main domain.
https://www.mail-tester.com/

Answer 2

To be honest, it's not entirely clear. Please, can you elaborate?