Outlook.com has quietly dropped[0] emails from certain domains/IPs for years, even with SPF/DKIM/DMARC all properly set up and no history of spam. As someone who's administering their own mail server, Outlook has been the bane of my existence.
[0]: As in: They accepted the email just fine (according to the logs) but it still didn't end up in the recipient's mailbox.
I went through this and ended up following these instructions (clearly AI-generated so strip through the verbosity) to get delivery restored: https://www.suped.com/learn/blocklists/how-can-i-resolve-mic...
It's probably just another bug in their software.