Determining Reason for High Confidence Spam

Question

Hi there!

I have a web system that for years has sent transactional emails to the users of the system. Within the last week, Outlook has all of the sudden started marking all emails from my system as High Confidence Spam, which has resulted in the majority of email addresses serviced by Outlook, to not have the email make it into the inbox (not even in the Spam/Junk folder).

I have utilized the De-Listing page to request my IP be de-listed. I have also submitted a Support Ticket via https://olcsupport.office.com/ but haven't received a response.

The headers are only showing Outlook is marking my emails as High Confidence Spam, but unfortunately have no idea how to tell why those emails are being marked that way.

Could anyone point me in the right direction on how to figure out why the emails are being marked High Confidence Spam so I can make any necessary adjustments, or how I can get in contact with someone that could help?

Thank you