Starting at 8 January 2024 several regions of Microsoft's outlook.com mail services are listed on the famouse RBL spamcop.
Here the details:
This mean that this blacklist spamcop, does not allow the incoming email from outlook.com due the listing.
We, as most of the mail provider around the globe, use spamcop RBL.
For now, we have temporary disabled the spamcop RBL check, but it's important to understand the this flaws should be resolved as soon as possible from Microsoft's side.
We remains at your disposal if you need more information on that matter.