Why do emails from mail clients go to spam?

Question

Fight against spam

Hello.

All letters from my mail server come correctly to all recipients on yandex, mail or gmail

As soon as I connect my server to Thunderbird or Outlook mail clients, all letters to the same recipients go to spam. Why is that?

Despite the fact that neither yandex nor google gives any errors

Answer 1

mail.favorit812.com
Reverse DNS is not a valid Hostname

Answer 2

Alexander, all letters come from the server correctly. What needs to be done so that they come from mail clients as well?