I'm told that the existing apps will get a minor UI update later this year to bring them in-line with other UI updates going on with Windows 10's big Sun Valley update, but the apps themselves will eventually be replaced by Monarch. The Mail & Calendar apps on Windows 10 are in maintenance mode between now and when the Monarch client is ready to replace them. Of course, that won't provide you with all the integrations Microsoft has planned for the Monarch client, but it does give a general idea of the kind of experiences you can expect to have when using it. Those curious about how this experience will look and feel can install the Outlook Web app via your web browser today. Microsoft also wants to replace the legacy Win32 Outlook client, but that is a goal which appears to be much further out and will be a gradual process as the legacy client is a mammoth that is not easily replaced. Microsoft will begin to preview its new Monarch client towards the end of this year, with plans to replace the Mail & Calendar apps on Windows 10 sometime in 2022.