How to properly organize email notifications and mailing lists?

Question

Email

Greetings!
There is a project with a large number of all kinds of notifications / mailings / reminders by mail, which are sent to users of the site and not only. I put the sender [email protected] . For example, users send information to the mail that they found on the site or 3 times a week notifications are sent about the expiring service.
I noticed that sometimes letters go to "spam", and sometimes they do not reach the addressee at all. :( Spam is not sent, all the information is necessary and useful for the user.
How to organize such mailings correctly? What does Yandex / mail focus on when throwing letters into spam?
Thank you.

Answer 1

Here are reputation recommendations:
https://habrahabr.ru/company/mailru/blog/216535/
Here are technical recommendations:
https://habrahabr.ru/company/mailru/blog/239963/

Answer 2

A lot of what focuses on:
The presence of the IP server in spam databases.
The presence of the server IP in the SPF record for the domain.
Match IP -> PTR record -> IP
Text/Subject.
Clicks by their users "this is spam".
DKIM.