In May 2018, Microsoft will retire the OWA (Outlook Web App) mobile apps on iPhone, iPad, and Android. Outlook users will find that it's easy to transfer their content and contacts from OWA to Outlook. As noted in a post on the Microsoft blog, Microsoft has opted to focus more heavily on improving the Outlook user experience on iOS and Android.
As of April 2018, OWA will no longer be available in Apple's iTunes or the Google Play Store. OWA mobile app users that also be reminded of the end of OWA functionality with links to download the Outlook app for iOS or Android. On May 15, 2018, OWA mobile apps will stop working completely on both platforms. At that time, Office 365 users will be redirected to a link to download Outlook on iOS or Android, depending on their operating system.
Office 365 users using Windows 10 and regular Windows 10 users will not be affected by the change, as they can still enjoy Outlook as usual. Office 365 administrators will receive a notification in the Office 365 Message Center, but they can also use Microsoft's instructions available here. Detailed technical information regarding the retirement of OWA apps is available here.
Currently, Outlook is a highly-rated email app in both the Apple iTunes and Google Play Store.