On May 15, 2018, the OWAs (Online Web Application) will stop working.
For several years now, Microsoft has been concentrating on introducing more features and neglecting their OWAs.
In April, the OWA will no longer be available for download from the iTunes store and Google Play store. OWA mobile app users will be (repeatedly) notified of the date when the app will be retired, with a link to download Outlook.
On May 2018, the OWA mobile apps will stop working completely. Attempting to open them will display a link to download the native app.