Autodiscover Ignores Exchange Online? Check the Registry
Outlook asks for Office 365 credentials while the mailbox is still on Exchange on-premises: set ExcludeExplicitO365Endpoint under the Autodiscover key.
During a migration from an Onpremise Exchange to O365 i encountered the issue that Outlook would ask for the credentials from O365. Because the mailbox was still on the Onpremise Exchange Outlook could not find a mailbox.
To resolve this we need to run the following Powershell:
Set-ItemProperty -Path “HKCU:\Software\Microsoft\Office\16.0\Outlook\AutoDiscover” -Name ‘ExcludeExplicitO365Endpoint’ -Value 1 -Type DWORD –Force
Set-ItemProperty -Path “HKCU:\Software\Policies\Microsoft\Office\16.0\Outlook\AutoDiscover” -Name ‘ExcludeExplicitO365Endpoint’ -Value 1 -Type DWORD –Force
After this you need to close Outlook and open it again, sometimes it can take a few minutes.