My experience is, that the „quality“ of the mail server‘s IP really matters. The worst experience I got was with digital ocean. A lot of providers just don’t accept email from their IP ranges. Some of them just completely block all DO IPs on router level, and refuse unblocking.

For my current server I had to switch IPs a few times, until I got one that was not blocked by any of the major providers. Unblocking a once blacklisted IP seems to be practically impossible.

And hotmail or outlook.com just mark a lot of email as spam. I see it now as a problem of the recipients. Office365 just accepts the same emails, it seems to be a strategy of the free mail providers, to give their non-paying customers a worse experience.