Fix for Outlook 2016, 2019, 365 prompting for a password when adding a second mailbox in Exchange Online, when the primary mailbox is still on-premises.
A common issue when migrating to Office 365/Exchange Online from on-premises versions of Exchange is users being prompted for passwords when connecting to a second mailbox hosted online, when their mailbox is still hosted on-premises.
This can happen in these scenarios:
- A users mailbox is on-premises, and they have access to another user or shared mailbox which is already in Exchange Online
- A remote archive has been created for the user using e.g. Enable-Mailbox -RemoteArchive (see https://cloudrun.co.uk/exchange-online/configure-exchange-online-archiving-for-all-users/)
In this case, Outlook will prompt for a password when trying to connect to the online mailbox.
- HKEY_CURRENT_USER\Software\Microsoft\Exchange
- New, DWORD Value.
- Name: AlwaysUseMSOAuthForAutoDiscover
- Value: 1
See the following Microsoft article for more information:
Outlook prompts for password when Modern Authentication is enabled
