How to avoid getting mailings in spam?

Question

PHP

Good day!
There is a server on which the frontend and web service for the iOS version of the application are running. The main idea of ​​the app is to send out some content every day. Since this content is filled through the admin panel, it is stored in the database of the same server. Plus there is a bit of logic, i.e. the system takes into account the number of the last mailing, it is possible to pause the mailing, etc. (I mean, just switching to a mailing service is not the easiest option).
I'm wondering why emails sent using PHP (the mail function) can end up in spam? The system administrator claims that 100,000 emails per day is not a problem for the server, but emails will still end up in spam, because some users will hit the spam button. Here you can register feedback loops. Are there any other reasons for getting there and how to eliminate them?
Thanks in advance!

Answer 1

The simplest filter for mailers is the number of letters in one IP, for Yandex, after a thousand letters per hour, letters begin to fall into spam, for Yahoo, after two thousand letters.

Answer 2

First of all, follow all the recommendations of the mailers directly:
support.google.com/mail/bin/answer.py?hl=ru&answer=81126
help.yandex.ru/mail/?id=1113150

Answer 3

I advise you to read - go.sendgrid.com/DeliverabilityGuide.html

Answer 4

1) Follow all recommendations of mailers, including List-unsubscribe, DNS, SPF settings.
2) Send letters only to those users who asked for it themselves (and not for whom they put a tick or wrote in small letters in the corner of the page). Decent services first of all ask you to confirm whether the user needs a mailing list, and only then they send spam.