I've tried reaching Microsoft by phone, email, automated tools for email administrators, and their web forms for requesting whitelisting in email deliverability. Microsoft didn't provide any rational why they are filtering email from small email providers like this, nor did they provide any opportunity to mitigate their filtering.

You can create anonymous servers on DO with go-betweens like Bithost, allowing you to pay with cryptocurrency. This is a pretty easy solution for people with bad intentions (bots, scraping, black hat ...)

I run my own VPN on DO and almost always get hit by ReCaptcha. Even Google.com will put up a "we've noticed some unusual traffic from" page. Some sites like Zalando completely block me with just a plaintext error page.