Why did the letters stop coming to the test box?

Question

Mail server

Good afternoon!
Tested sending email notifications. Notifications by agreement with recipients. The scheme is like this. From excel, the data is sent to the site, which sends them by mail to a specific list of recipients. In battle, while the letters reach. On the test, when I tried the new logic and load, there were several sends to the mail.ru test mailbox. The volume of each of the shipments was much more than the daily one. At some point, the letters sent by the site stopped coming. At the same time, all the data to send is successfully received on the site. The message is sent every 2 seconds. I suspect that the box from which the letters leave could get into a spam list. At the same time, in the settings of the test box, the spam list is empty, the sender box has been added to the contact list.

Tell me, what is the nature of the phenomenon? Is it somehow possible to fix it from the recipient's side? What can be read on this topic?

UPD: I ran into similar situations a couple of times. For each of them individually, all answers were correct. One solution worked everywhere: moving to your own hosting.

Answer 1

mail.ru considered the sender a spammer (which is very similar) and banned him.