I've had this exact problem for years. My IP addresses have been used for 15+ years for sending e-mail, they are spam-free, but Microsoft keeps blocking them. Every two months or so I have to ask them to unblock the IP again, then I can send mails to Outlook again, until they just random decide to block me again. It's an absolute clown show.

This is the price every small sender pays. The unblock request process is essentially designed to make you give up or move to a large ESP. There's no appeals process, no SLA, no acknowledgment that your reputation data might just be wrong. You're at the mercy of a system that treats false positives as acceptable damage.