See also (recent): https://answers.microsoft.com/en-us/outlook_com/forum/all/ho...
Imagine if this was turned around, and simultaneously a huge chunk of the world instantly black-holed Office 365 outbound email.
Then in response to the roaring noise from Redmond, simply send back their canned response of "Not qualified for mitigation. Our investigation has determined that the above IP(s) do not qualify for mitigation."
Do that for 24 hours, then 48 hours, and so on until they smarten up.
I guarantee you they'll have their CEO breathing down the necks of every moron in the spam filter team until they fix it.
Until then? Nothing will happen. Nothing at all. We'll complain on YC News, we'll complain on /r/sysadmin, and a bunch of small businesses will go out of business. Microsoft 365 will continue to grow to consume the world. Email will be rebranded "Teams Mail", and as far as everyone is concerned that will make perfect sense.
Either we act, or we live with the consequences of inaction.
Yes, email is kind of important. But it's also pretty shoddy. It always has been. (Confidentiality, integrity, authentication, signed receipt of delivery. All are important for regular business correspondence and by default all were [and still are] hit and miss with email.)
This practice of MS of not accepting email might be abuse of market power. But they can easily say their clients signed up for Outlook to have exactly this protection. What would the courts think? Who knows.