Same experience here. We have multiple customers whose servers are in large blocked IP ranges, with seemingly no way to get them delisted. The strange thing is that Microsoft only seems to reject their emails on Hotmail/Outlook.com accounts (free accounts), while accepting them just fine on business accounts. Meanwhile, their “support” tools tell you there’s nothing wrong.

Based on my broad experience, I’d comfortably rank Microsoft as the worst of the major email providers. (I am owner poste.io)