What is the correct spf and dkim for your domain/s?

Question

Domain Name System

There is a mail server on Postfix and several domains that it serves. Everything is working. Only spf and dkim are not configured yet. I've read about them, but I don't quite understand.
Let's assume that I have:
postfix_domain1.com
and 3 domains that use it:
email_domain{1, 2, 3}.com
Explain in a simple and this specific example how to set up these records?

Answer 1

1. spf - there is no such type of records on the DNS server, since there was no spam when the RFC was created, so this is a crutch and it is written in a TXT record.
Just write the appropriate entry in the DNS zone editor,
something like this:
Listing EVERYTHING! ip from which mail can go, especially if these are mail services like Yandex mail, since mail can go both from Yandex and from your server.
With DKIM and DMARC it’s more difficult
https://habr.com/post/343128/
You will need to generate an ssl certificate on the server, moreover, using the opendkim daemon,
and enter the public key into the same TXT record, but with your own
DMARK syntax, just a record, but only when working. Here you
can check
https://mxtoolbox.com/spf.aspx
https://mxtoolbox.com/dkim.aspx
https://mxtoolbox.com/DMARC.aspx
its updates.
You can see if the updates have passed with the utility
host -a test.ru(it is available in the deb branch by default)