We apologize for the inconvenience. We currently don’t support the ability to open shared mailboxes or other mailboxes that you have permissions to access.
To open these mailboxes, you can access Outlook from a desktop browser (by requesting the desktop site in the browser settings) or use the Outlook mobile app that can be downloaded from the iOS and Android app store.
An error occurred while saving the commentAndreas Link commented
this function was still available until recently. There is no announcement so this should be classified as an error and should be reported to the development department for error correction "as mandatory" since this function is used by 80% of business customers. So please take care of it!