https://tuta.com/blog/outlook-falsely-marks-tutanota-emails-as-junk

"Since mid November, our users are reporting that their emails sent to Outlook are being flagged as Spam. This affects all emails sent from any tutanota.com email address. Our team reached out to Microsoft’s support team as well as to our direct contact, a board member of Microsoft Germany. However, the issue has yet to be resolved."