Users who still have their mailbox in Legacy Microsoft Office 365 dedicated but use the Office365 portal for other workloads may see the Mail, Calendar, People, and Tasks application tiles in the application drawer. When they click the Mail, Calendar, People, or Tasks application tiles, they are directed to a page that states that the mailbox could not be found:
In a migration from legacy Office 365 dedicated to Office 365 vNext, the user objects in vNext are granted an Exchange Online license. This is expected.
After the Exchange license is applied, the Mail, Calendar, People, and Tasks icons are displayed in the application launcher. However, the mailbox is still located in the legacy environment. Therefore, these applications cannot be located in the vNext environment.
Contact your EXO vNext Release Manager and your Service Delivery Manager (SDM) to request that a redirect URL be applied to your environment.
After the redirect is implemented, when the legacy user clicks the Mail, Calendar, People, or Tasks application, a page appears that provides the correct URL: