Just had a friend reach out yesterday about this issue. His outlook account for 10+ years started having issues receiving emails from his dad and a company he works with.

All I could find was that his dad’s email was missing SPF/DMARC but the other email address that was having problems looked like it was configured correctly.

I only was able to get a screenshot of the email voice his dad received and it mentioned being on a block list (like in the article).