How to make sure that letters from your own mail on your domain do not fall into spam?

Question

Domain Name System

The question is basically generalized, since I don’t even know in which direction to dig.
I checked the DNS, everything seems to be fine and there are no errors in the mail itself.
But at the same time, the letter 100% falls into spam.
The letter itself is sent automatically from the server, for example, to activate an account with a key.
The letter itself contains an unsubscribe form and does not have pictures or suspicious words.
In general, where to dig and what to check specifically? And in general, is there any service from Google to add mail to the white list and check the settings?

Answer 1

SPF, DKIM, DMARC. Well, do not send spam from your server.
You can check at https://www.mail-tester.com/

Answer 2

What to do was answered above.
You need to add another PTR DNS record to SPF, DKIM, DMARC.
Many public mail servers have services - usually called postmaster.
Google has it https://postmaster.google.com/