What to do if letters through Yandex SMTP start to fall into spam?

Question

PHP

I constantly develop websites and always use mail for the domain from Yandex to send letters from the site.
The domain is delegated to Yandex, then a no-reply mailbox is created and mail is sent from the site via SMTP.
What kind of letters? Registration notifications and letters from feedback forms.
Recently, on every new project, a problem with this has begun to arise. I get the following message: Expected response code 250 but got code "554", with message "554 5.7.1
[2] Message rejected under suspicion of SPAM; https://ya.cc/1IrBc

24 hours (without sending anything at all.) Nothing helps, but 1 email out of 100 is still sent.
How to deal with it? Maybe something has changed in the rules of Yandex?

By the way, I tried to implement Google through smtp, the same problem.
Letters are usually in the following format:

Пришла новая заявка в форму обратной связи.
Имя: Иван
Телефон: [phone]

Moreover, it is important that on old projects the same mailing list works fine and does not produce such errors.

Is someone experiencing similar problems? How to fight? What to use to send emails from the site (free) ?

Answer 1

What to use to send emails from the site (free)?

SendGrid, Mailjet, Mailgun.
But the latter, it seems, either removed the free plan, or changed the conditions for receiving it. Previously, it was definitely, in my opinion, for 10,000 letters.
The free option of sending from Google Cloud has been preserved: https://www.mailgun.com/google/
Cheap ones include Amazon SES and Elastic Email.

Answer 2

Read here, just this topic
https://7fk.ru/nastroyka-smtp/