Ask HN: How can I get my mailserver to deliver to Hotmail?

I'm pretty desperate since my VPS mailserver (on DigitalOcean) does not deliver emails to any of Microsoft's services (Hotmail, Live, Outlook). This is my company's email server and I've been running it for two years, it only sends legit personal emails, not even "drip campaigns" or such. Their mx reports that the mail has been delivered, but it does not appear in the recipient's inbox or spam folder. It is simply lost and I can't even monitor delivery errors. Sometimes a message goes through (usually a direct reply to an incoming mail) but that's not even guaranteed. I've completed all checklists, my mailserver uses DKIM, SPF, in-transit TLS, does not send spam, has a reverse PTR, etc, etc. The IP is not listed in any blacklist. I signed up for Microsoft's postmaster tools and IP validation services. I get a perfect 10/10 score in most mail tester services. Other MXs like Gmail and Yahoo! deliver emails properly, sometimes to the spam folder but that is usually fixed when the recipient clicks on "Not spam". Contacting their support like is talking to the wall. It's impossible to get a non-canned response. Short of paying the extortion fee for a "deliverability certificate" I honestly don't know what else to do. I understand spam is a serious issue but come on, how are small businesses supposed to send emails if they don't want a third party like Google Apps to read them? Any ideas, HN?