Since December 4, 2019, the "Outlook Android App" can no longer communicate with Exchange servers in companies.
Unfortunately there is neither a remedy nor a reaction on the part of Microsoft.
For details please see the following link. There, administrators from all over the world have already described the error in detail:
https://answers.microsoft.com/en-us/msoffice/forum/msoffice_outlook-mso_amobile-mso_mobapps/503-errors-in-outlook-android-app/a107dff2-9f47-41a9-8e13-b1b7204e84aa?messageId=8d9155bf-4cd9-4219-b170-9da1952438cc
503 Errors in Outlook Android app
