I bet Microsoft is single-handedly responsible for 80+% of complaints about email being impossible to host.
In a federated / pseudo-decentralized network, any provider with 10+% of captive users has an outsized leverage on the entire ecosystem. You don't need a monopoly; as long as you have a monopoly on just a few users that others absolutely need to be able to interact with, you can dictate terms to everybody.
They are the reason I dropped self-hosting my mails. They were silently dropping some of my mails depending on the day I sent them. If they were at least sent to the spam folder, it would be easier to communicate with their users. And since a lot of their users are not technical at all, it's complicated to explain them the issue.
It's absurd in every way. I was trying to get it to stop spamming maillist traffic. It's impossible. Global level filters enacted as admin to not spam check are ignored.