Send mail directly or indirectly?

Question

Mail server

Folks, I don't know much about Postfix at the moment. Almost all my letters end up in spam (or worse, Outlook doesn’t even send it there), I read on the official postfix website that sending without smpt (directly, not through yandex / google) is either impossible or undesirable. Or do I not understand correctly (I scored in the translator, I know English as an amateur)?