I know people are saying it’s your isp and that is a possibility but IMO ms is being too harsh here. First let me say at least they have a support department unlike gmail. As for your isp I honestly don’t get a significant amount of spam from digital ocean or many of these vps systems so I don’t think it fair to say that is the problem right away ( this is spam only not ssh brute force scanning which I don’t track ). I get significantly more spam from aws, gmail and Microsoft itself - I’m personally annoyed that these providers are too big to block but have no issues blocking other ips with no ability to resolve it. Sorry I have nothing to add to help resolve this. A month ago there was a post on a similar issue but with gmail. At this rate the response will be the problem is you are running your own email server and should pay for delivery.