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.
This is with a "real" mail server, and not mailu.io, but the idea is the same.
My IP is sparkling clean for many years now, dkim/spf etc, but gets blocked on any MS mail server. Tried appealing and heard nothing whatsoever.
You mean global monopolies, for which there is no legislation for. Ergo the US Govt is holding the rest of the world hostage via its tech companies.