Microsoft Exchange Online Authentication failing, Error Code: CAA2000B

So, something went wrong [4usqa] on some Microsoft Exchange Online tenants. Authentications are failing for Outlook Classic users, and they are kicked out of their session. When attempting to log back in, we are greeted with the following notification

and this…

Authenticating and OWA session on a browser works fine, so no issue with the subscription.

So until the time this post is published, no one knows why this is happening. My assumption is that someone at Microsoft, flicked a switch to activate new features or something, but ends up screwing their tenants

Here’s how you can get your service back:

1. Log into Microsoft Entra
2. On the left pane of Entra’s home page, select Applications, and then Enterprise Applications
3. Remove “Application type == Enterprise Applications” filter
   
4. Search for “Microsoft Information Protection API” on the search bar under Enterprise Application
   
5. Click on it to load Microsoft Information Protection API’s overview pane, and click on Properties
   
6. On the Properties page, change “Enabled for users to sign-in?” from “No” to “Yes” and click on the “Save” button to err.. Save..
   

Your Microsoft Outlook Classics users should now have access to their mailbox again!

…too bad, eh?