Outlook 2016, 2019, 365 prompting for a password when adding a second mailbox in Exchange Online, with the primary mailbox still on-premises.
This can happen in these scenarios:
- A user’s mailbox is on-premises, and they have access to another user or shared mailbox which has already been moved to Exchange Online
- A remote archive has been created for the user using e.g. Enable-Mailbox -RemoteArchive
In this case, Outlook will prompt for a password when trying to connect to the online mailbox.
Fix it, by adding the following registry key:
- New, DWORD (32-bit) Value.
- Name: AlwaysUseMSOAuthForAutoDiscover
- Value: 1
This will force Outlook, to always use Microsoft Office 365 Authentication, for Autodiscover, read more here: