Yeah, I didn't want to get stuck in details, but I'm setting up a couple of new mailservers (freshly registered domains, self-hosted VPS at cheap cloud provider

Yeah, I didn't want to get stuck in details, but I'm setting up a couple of new mailservers (freshly registered domains, self-hosted VPS at cheap cloud providers (DO, Hetzner, OVH, AWS). And they do get delivered directly into Outlook, Google, Protonmail and other major providers after two days tops (some still employ greylisting for fresh IPs/domains). There's no secret sauce but there is a myriad of ways to shoot yourself in the foot when setting up a mail server from scratch. E.g. many standard setups may try delivering email over IPv6 if available which currently is a terrible idea but might be completely opaque to the user. Obviously, the results will also depend on what is it that you're sending as everyone uses point-based (or even ML) systems nowadays and headers + contents can earn you either negative or positive points.